The term gimbal refers to a point of rotation. The purpose of a camera gimbal is to counter the rotation of the multicopter as it tilts for flight by rotating the camera in the opposite direction on the gimbal. This effectively cancels the rotation of the multicopter and stabilizes the camera. The following image illustrates how the camera remains level even while the multicopter tilts:

So, that takes care of pitch and roll … what about yaw? Yaw works a little differently. Some gimbals only stabilize on two axes (pitch and roll) so that FPV (first person view) flyers can tell which direction they are headed. Older 3-axis gimbals left the camera pointed in one yaw direction, no matter where the multicopter was facing. This could be problematic and confusing if it was flying using FPV. It would be like the cockpit of a helicopter pointed north while the rest of the aircraft faced south. Then, do you push the stick forward to go forward? Or would that make you go...