Lions are the main currency in the economy of War Thunder.  You can earn Lions by destroying planes, ground targets, or by completing achievments and challenges.

The Lions can be used for purchasing new planes, repairing planes, buying new weapons, or modifications.

It's possible to buy Lions with Golden Eagles (the premium currency of War Thunder).

It is vital however, to keep in mind that all vehicles and previous vehicles (excluding additional and optional variants, premium vehicles, and vehicles in other trees, e.i Fighter, Bomber etc) must be researched in order to purchase the vehicle. in order to research vehicles you may either complete missions or battles and gain research points for the vehicle.