In gamer-speak, any action that prevents the free movement of another player is referred to as "blocking." More specifically, standing still in narrow corridors, on stairs, or near corners will invoke the wrath of your teammates and angry imperatives to "quit blocking, noob."

In order for blocking to take place, a few things must happen. First, the game must have some form of player-model collision. If players can walk through each other, then the only blocking that will be going on is visual. Second, blocking usually occurs in team games. In a lone wolf deathmatch-type game, blocking is much less likely to occur, as players have every reason to shoot the offender rather than try to navigate around them.

Blocking can occur for several reasons. One of the more common is the fact that many people simply don't think in a team-oriented fashion, and don't realize that when they stop in the middle of a hallway to return fire, they're preventing their teammates from reaching much-needed cover.
Another common reason is that the player who is blocking is doing so on purpose, either because he's an all-around jerk, or because he has tired of playing on this particular server and has decided to make life difficult for his teammates before he parts.

Whatever form it takes, blocking is an unfortunate occurance that has irritated many players since gaming's inception, all the more frustrating because of the fact that there isn't much anyone can do about it, aside from team-killing, which is frowned on as much as or more than blocking.