To someone just starting out, you might not think twice about where you choose to get your camera angles from. As long as it looks good, right? Well, not quite. Just because a shot looks good doesn't mean it works correctly with the scene. The 180 degree rule is what helps us ensure each shot in your scene works correctly with one another.
The basic principle is this:
if you have two actors interacting, draw a line through them, this is the line you cannot cross, if you shoot one actor from one side, you should not move to the other side when shooting the other actor's angle. The idea here is that on screen, actors should always be facing each other. So if actor A is facing left to right in the frame, actor B should be facing right to left. This principle applies even if you have more than two actors, the line is determines by who is facing who, and them turning their heads to face one another is the key to establishing that line shift.
Exceptions to the rule:
Now, this rule is not set in stone and CAN be broken in numerous ways which are legitimate. Sometimes people break it to specifically confuse or unsettle the audience. If you have a slider or dolly, you can cross that line in a camera movement, you've seen shots in movies where the camera dollies behind an actor. Once on that other side, it doesn't mean you cannot now use both sides, you must now maintain the rule from that side.
Those are the raw basics, the two videos posted go into even more depth, and both take different approaches, i recommend watching both of them even though they cover the same topic.