FPS Frames-per-second. An indication of how well your hardware is rendering the game.
Lag Delayed responsiveness.
Mod 1) Abbreviation of moderator. 2) A mod, custom content for Minecraft, often assembled into collections.
Reconnect Disconnect and then rejoin the server.
Relaunch Exit out of the game, and relaunch the entire program from the launcher.
Relog A.k.a. "relaunch".
TPS Ticks-per-second. An indication of how well the server is processing the game. Ideal is 20.
Vanilla Unmodded, original Minecraft.
VIP VIP rank, usually puchased.