First of all, you'll probably find
the wiki to be a huge help in understanding various aspects of the game.
To directly answer your question, the tech tree is the progression along each vehicle class for a given nationality. You can only progress through the tree by acquiring experience, or "XP", an then "spending" it to research a given vehicle or to upgrade the various modules of that vehicle, such as the engine, radio, gun, etc.
In addition to earning experience during battles, you also acquire credits. Once you have researched a vehicle using your XP, you can then buy the vehicle with credits. Modules also cost credits.
In short, anything that you acquire in the game requires a combination of experience to "unlock" (research) and credits to purchase. The exception are some items, such as premium ammo, consumables, and tanks, that you can acquire with
gold without the need to unlock.