The distance (height) that we can climb in one go is limited by the length of our rope. When we reach the end to the rope we need to stop and start again.  More usually, we stop well before the end of the rope on a suitable ledge. Either way, when the leader stops climbing to bring their second up to join them, we refer to this as "a pitch".
On small outcrops of rock, the height of the climb is much shorter than the length of our rope and we can climb to the top of the climb in one go. This is a "single-pitch climb".
However, on a very large cliff, we need to climb in repeated cycles of the leader first, then the second (one pitch); then the leader first again, then the second again (another pitch); and so on until we reach the top. We call this a "multi-pitch climb." Even if the climb has only two pitches, it is still a "multi-pitch climb".
Additional knowledge and skills are needed to handle this transition part way up a climb. There are hidden risks that a novice would not be aware of. There are also lots of tips and techniques to make this transition smooth and efficient, otherwise the climb may take more time than we expect (or have available).