Vibration is mechanical oscillation (regular or otherwise) about some equilibrium — prone to becoming problematic in motor-driven motion designs where there’s looseness, backlash, windup, uneven effects from friction, machine-assembly imbalances, or shock loading.
The ultimate goal for design engineers is to build and tune designs for minimal vibration (free or forced) and render unavoidable system vibration damped (dissipated) or isolated. That way the machine can quickly settle back to equilibrium.