Wipe: Tarkov is still in beta, as such balancing, leveling and the economy aren't worked out yet, so periodically they reset the game to square one: you lose your progress, along with everyone else, and have to start from the beginning. This is called a wipe. (at first I thought this would be annoying, but many players find it exciting, it renews the game, and early stage and late stage wipe play very differently.)

Flea and meta: the flea is a market between players instead of only having npc stores. It changed the way the game works, as people with a lot of money can always have meta loadouts. meta in this context refers to the best performing configuration of items, so the meta is x weapon with y ammo etc. (meta is a common term: for example in overwatch the various heroed are dubbed in the meta or out of the meta depending on the latest game mechanics updates)