You know how in a play, you only ever see the action from one side of the stage (barring any weird performances, of course)? Film and TV work the same way. At least, in theory. When you have a scene, all the shots should be taken from the same side of the action. For example, if the scene is focusing on two people on a couch, you wouldn't suddenly have a shot from behind the couch (or, if you simply must, then at least don't confuse things further by adding in shots from the sides and so on). The imaginary line that marks the boundaries of where the camera can be placed is called the axis, and you do not ever cross it.

It is especially important because it is related to the grand force of continuity. If you've got a guy hanging around on one side of the screen, it's just bad form to suddenly have him on the other side, though he hasn't physically moved at all. It's disorienting, especially if you don't have anything that could establish a need for the camera to move (seriously, you can forgive almost anything if you have some footage that allows for establishment).

This I have learned from the film people I know and love.