This is a very common issue known as abrasion when the patient is brushing their in the incorrect technique , with an incorrect brush /toothpaste for a long time , and this is more common than people reaslise. As you can see the yellow colour dentin is exposed and the teeth can have sensitivity , and also dentin is not very strong and so can come away easily and so I would recommend abrasion cavities be filled witha white filling to avoid further damage , and potential tooth loss as if it is a tooth the front of the mouth I would strongly recommend this be filled. Simple dentistry done well.