Every piece of rotating equipment/machine vibrates while it operates. Some level of vibration, or oscillation, is completely normal. But when an machine becomes unbalanced, develops wear and tear, or has a bearing fault, its vibration levels will change. The machine may start to vibrate at a higher frequency, or the level of displacement may increase.
A high-quality vibration meter can pick up tiny changes in vibration levels that human senses would not notice. Those tiny changes are usually the first indicators of a developing fault in the equipment. The sooner the fault is detected, the more easily the maintenance team can correct the problem.