In short words: They're additional upgrades to your tank and crew. You don't really need to get acquainted with all of them. Some of are just better than others.
Bonds are a third piece of currency, it can only be earned by earning battle medals - higher rewards on higher tiers. The only directive that essentially pays for itself is the FF directive - it's not the best by any means but this is the one you can run every game on high tiers while still making profits.
Vent, Rammer, Vstab directives are still king in general as primary benefits for most tanks, but some tanks with very unique traits now have the freedom to venture outside of the same old loadout. I wouldn't use any other than these on anything primarily intended to deal damage, but to each his own. The good ones obviously cost more. This is a system more reminiscent of the old silver system, where maxed loadouts simply aren't sustainable. Some directives can be purchased with credits, most of them are rarely useful though.
Bonds can be earned in randoms through medals, ranked battle season placements and clan wars. Grand Battles (while rare, even when enabled) grant you more. It's a 30v30 map, so things are different. Prepare to have 5 arties shoot at you all at once again. Some missions also grant you bond rewards. Season Passes seem to do similar things, they also contain the Bounty equipment - which is the most valuable pieces of equipment you'll get your hands on in this game.
The most efficient way of spending bonds is to fill the gaps with whatever bounty equipment gaps you can't fill. 10 gold dismounts is much easier to manage than 200 bonds for demounting. This means skipping directives and letting the competitive edge go until you at least have one full set that you can rotate. Bounty equipment is improved equipment that can be de-mounted with 10 gold, like regular equipment. Improved Equipment (has identical perks as the other) costs bonds. Even here you should be smart. Buy the bundles. Saving 1k bonds in the purchase of 2 modules saves you many, many hours of grinding. As you complete your sets - put them on tanks you're going to keep playing. CW meta tanks if you're into that, meta tanks for randoms if that's your thing and so on. The idea is to move your bounty equipment to enhance your edge on vehicles you're only going to play periodically. Same goes for the purchase of directives, once you decide to go full tryhard - save uip and buy the bundles that'll cut your rate. It's a tedious process but it's the right way to go. I have 8 full sets of Improved Equipment and all available Bounty Equipment - meaning that I can move those around for gold and keep my bond economy as solid as possible. A 100 game tryhard with 100 vent directives isn't exactly cheap - you're going to burn through them at a higher rate than you can be paying them regardless of how skilled you are at the game. There's no workaround. Pile it up and focus on equipment. The new equipment pieces don't have improved versions - so they rarely compete. A fully kitted RU med in every sense of the word against someone running standard equipment will greatly reduce their chances. A 907 drops 0.2 seconds of reload meaning that against an identical enemy you will be a full second ahead in RoF after 5 shots. When it comes down to dogfighting - this really is an advantage that's hard to ignore.
If you are talking about the campaign missions - you should focus on the 260 ones first as these all have the same condition; complete it in a single game. The 2nd campaign is far more tedious - as you're not only going to need luck once, but several times in a string of events that after a while - will start to get on your nerves. Rewards are good though, the amount of bonds through these and season passes, and a ranked season - nets you quite a bit. A full equipment set costs 14000 bonds, and with grade A directives alongside it - you're about to spend a very long time to give you a window of absolute competition. Nobody's got better stats on the vehicle. More often than not, you go up against standard equipment. All you need to do in a clean 1v1 is to keep firing to win it.
Do it smart. Save up - equipment pieces first. Kit out the one tank you know you're not going to stop playing. Wait for season passes to come around, even if you don't buy them - the bounty equipment is still up for grabs. Snag that whenever you can, I'd wager that Bounty Ventilation is worth much more than a regular piece. 5000 bonds is probably worth around 20 million credits as you can grind up that amount of silver in the same amount of time. This is the one currency that you do not make mistakes with, don't mount these pieces unless they're going to sit there for long enough to warrant the 600 bond demount cost for all of them.
You're quite a bit behind in this, especially on a new account now. It'll take a while to gather as the easiest way to complete a full set is to partake in the ranked seasons and do well enough. 3 pieces can be acquaired in 2 phases. It's a grind but your tank is going to feel a lot more comfortable maxed out. M60 literally turns into a railgun with zero bloom when equipped right - which is not the case with regular equipment. Other examples aren't as drastic, but the higher you can stack your bonuses - the better chance of you coming out of the fight winning. That's how it works now.
Use bounty equipment for grinding, it's as cheap as demounting regular equipment so those are the ones that you can move around more frequently without having to spend so much time when you want to use it on things you'll only be playing for a set amount of time.
Favourite tanks first - Viable tanks next. Bounty stuff can be used during grinds, but spending 600 bonds isn't worth spending on something you won't be playing for less than at least 2-300 games more. If you want to tryhard then getting the equipment pieces is the first thing to do on the list, then some spares for directives whenever you really want to tryhard. When that's set you can free up these costs as you won't be running around buying more equipment pieces when all of the relevant vehicles have them filled. That's when you bet big on directives for tryharding.