Gimbal lock happens when the middle ring rotates 90 degrees, causing 2 of the gimbals to become parallel, which “locks” the system into a degenerate two-dimensional space.
For example, if the pitch is rotated ±90°, then yaw and roll axes align, so changes to yaw and roll apply the same rotation. This means that the system loses one degree of freedom and can’t rotate around the remaining axis.
“thing of note to easily understand gimbal lock: imagine you are playing a first person shooter and you point your gun vertically at 90°: you are no longer able to point the gun left and right, moving the mouse sideways only rotates alongside the gun’s axis. You have two axes locked in the same attitude providing the same rotation.”