Gimbal lock is the phenomenon of two rotational axis of an object pointing in the same direction.
Simply put, it means your object won't rotate how you think it ought to rotate. Gimbal lock is frustrating problem that every CG artist will face sometime in their career and it always happens at the worst possible time. It's a general limitation of that type of rotational matrix. The reason for this is that Eular angles evaluate each axis independently in a set order. When that operation is complete it then travels down the Y axis, and finally the Z axis.