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)

meta is an acronym of Most Effective Tactic(-s) Available

haha, thats nice. I believe that's a backronym though.