AFK: Literally "Away From Keyboard". When a player stops playing for a certain period of time.
BRB: Literally "Be Right Back". When a player leave the server to rejoin soon in 1 - 2 minutes.
KICK: This is a command who only the admins of a server can apply to a player. The command makes the player in question leave the server in case he's AFK and he didn't write it in chat, or if the player in question makes spams or offend others players.
BAN: When a player is permanently throw out from a server. This commands can be used just by the admins of the servers and he's most of the time applied because of cheating or extreme offending.
CLUTCH: When the rest of your team is dead and you (or someone of your team) saves the round against the rest of the enemy team.
GJ: Literally "Good Job". This term can be used when someone (or you) did an amazing kill/saved the round.
GG: Literally "Good Game". When a game is finished, you can say "GG" if you wish in the public servers. It is used most of the times at the end of the clan wars.
GH: Literally "Good Half." Used by players after the end of the first half of a match in which the players switch sides.
GL HF: Literally "Good Luck Have Fun". Used by players prior to the start of a match to express admiration or good sportsmanship.
WALLBANG: In the Counter-Strike series there's the "wallbang", which is the killing of an enemy (or a team mate) shooting through the walls.
RUSH: Overwhelm defending players by having the whole team attack the same position in a coordinated attack.
OWN/PWN: To kill and/or dominate an enemy.
TEAM KILL/ TEAM KILLER: Used when someone of your team kills you or your team mates.
TEAM FLASH: When someone of your team throw you/ your team mates a flashbang. This makes a lot of players very angry.

/rank: Used to show your rank in a game. Not all the servers have this plugin installed, but most of the public may have it nowadays.
/me: Used to show how much damage you did to your opponents.
/hp: Used to show how much damage an opponent did to you when you died, with which weapon and how distant was him when he killed you.
/rtv: Literally "Rock The Vote". Used to choice the next map, if you're tired of the currently in which you're playing.
/voteban: To make a votation to ban the cheaters in the game or someone who's desturbing a lot.

ECO: Conserving cash during a round by purchasing cheap weapons, purchasing only grenades, or not purchasing any items to prepare for the next round.
NEWBIE: A friendly term which indicate someone new to the game.
NOOB: A derogative term indicating a very irritating player. It can be mistaken for Newbie, however, they are not the same.
OP: Literally "Overpowered". Usually referring to a weapon capable of dealing incredible damage with little effort.
NO SCOPE: Usually used for Sniper Rifles. It's used when someone kills an enemy without the scope of the sniper rifle.

If there's someone new to the game reading this guide, I hope this one helped you! rate up if it does/did!
P.S.: I copied 4 - 5 terms from site [counterstrike.wikia.com] because very hard to explain!
I also made another Counter-Strike guide, check it out if you wish!
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=431086073