Two different shots need to match up to create one smooth action.
Shot/reverse shot
Switching between characters to show the convocation between 2 people. So you film one person talking, then the other then back to the original speaker. Usually filmed by a shot over the shoulder.
180 degree rule
There is an invisible line the camera cannot cross when filming. So that the audience doesn't get confused and the same person is always on the left or right, they don't switch. Therefore you always have to stay on the same side of the invisible line when filming.