In 2D fighting games, the screen covers a part of the stage; most stages are limited horizontally, so when you approach the end of a stage, the camera will stop, and at some point, you will be unable to advance further that way.
This wall is known as the “corner”, because you or (hopefully) your opponent is stuck in that corner of the stage. You could also refer to the “wall” or “edge” if you like (or if it’s more appropriate to the situation/game).
If your back is to the corner, you won’t have that direction to escape to any longer, so it’s generally a disadvantage. Also, some combos that wouldn’t normally tie together due to the range getting too far can, because the pushback distance is decreased (with the cornered party being unable to be pushed backwards, the overall pushing away is decreased).
However, you can also contain all-out opponents with the corner, so other advantages may be given according to the game’s systems and the situation.
Some 2D games will zoom out in order to reveal more of the stage, so that there isn’t much of an edge to the screen unless the players are very far apart, or actually in the corner. Other 2D games have played with the concept of this edge, so that characters may bounce off it when certain moves hit.