The 30 degree rule is a basic rule in filmmaking that simply states that when you cut to a different shot you should move the camera by an angle of more than 30 degrees. This is because if you move it less than 30 degrees it will feel like the camera is jerking and puts too much focus on the camera instead of what’s going on in the scene, ultimately disconnecting your audience from the story. For example, imagine you are watching a movie and the actor is speaking to someone off camera, then the camera cuts to the same actor but the angle is just a few degrees different from the previous shot. It’ll look like a mistake and probably annoy your audience which is never a good thing.
Often in a scene, where there is dialog the camera would cut from one actor to another and even change the location sometimes. Fr example, it may start with a front shot of the actor, then switch to a side or back shot the next time the actor is in a scene. Also, make sure you keep your shooting style consistent.
The short video below from the International Academy of Film and Television gives a nice description of the 30 degree rule and describing why it’s one of the fundamental rules of filmmaking.