The meta of a game is the kind of inner game of the game. In league of legends or DOTA2, you have the latest additional champion, or maybe some heroes/champions got nerfed/buffed making a particular play style fall out of the meta and another play style emerges as more viable for the current update. Maybe a new type of champion is released that throws every current strategy for a loop and the whole game style shifts, it can be any number of things influencing the meta.
Counter strike global offensive meta is similar, recently the CZ-75 pistol got nerfed, it used to be an instant kill headshot automatic pistol that only cost $500, it was (before the patch) seen as a very over powered weapon for the price, high level competitive play would have an entire team of 5 buying all CZ-75's in the pistol/Eco rounds. Now after the nerf in the new update the CZ-75 is very ineffective, there are much better buy options for that price point. So the meta has shifted away from 5 man CZ-75 pistol rounds, it promotes using the TEC-9, the P250, the Deagle, or the Five-seven. Some people argue there should be more spots for wallbanging in CS:GO, more wall bang spots (shooting through thin walls) will drastically change the counter strike meta, new strategies will need to be thought up, new rotations, etc.
So while none of these things are the core games themselves, its another layer of information and mechanics that influence how the game will be played out.