The 180° rule is simple. When you film a scene, you draw a straight line that you can't cross with any of the cameras. For example, if you interview a person, the camera should be positioned on the same side of the room. The people in the interview look like they are talking with each other. If you move over that imaginary line, the people will be facing the same way when you cut your scenes which can be troubling for the viewer. On the drawing to the right, you can see that two cameras are on the same side of the imaginary red line. The third one (crossed out in red) would disorient the viewer. You can move your camera anywhere as long as you stay on the same side of the red line.

Like all rules, it can be broken. If you do it, make sure it is a conscious choice and do it for a purpose. 

Here is an example below.