A tick is like a snapshot of the game state, i.e. where each player is, what they are doing, where they are looking etc. The tick rate describes how often this game state is calculated by the server and sent back to the clients (our PC's).

The main reason why CS:GO 128tick server are so much better than 64tick server is because in Global Offensive the server doesn't know exactly when each player pressed a button. The server only knows the tick in which a button was pressed or a head was shot. This leads to a tiny bit of discrepancy between what you actually did and what the server thinks you did (e.g. you were on the head but the game thinks you missed since in a single tick the player moved out of your crosshair). 128tick makes this more accurate.

Btw. this is also the reason why we need jump throw binds for smokes, since the jump throw only works if both buttons are pressed in exactly the same tick. Not even pro players can do this consistently.

CS2 provides the server with the exact time at which you shot, at which time your crosshair was on the target and at time you pressed a button. This means the hit registration is exact and doesn't happen only every 16ms or 8ms on 128tick servers. Thus CS2 64tick hitreg > CS:GO 128tick. But there're other factors in play which decide how the game feels, which is why 128 tick still has (minor) advantages in CS2.

Hopefully that helped. If not, feel free to ask and I'll explain as far as I know. This topic is still developing and there's much misinformation floating around.

Buncha people think they know more about programming netcode than the developers.