If a small amount of food debris is left in your teeth, (usually happens when one misses night brushing/is careless with it) the bacteria which are present in the mouth react with the food. As a result, acid is produced. This acid starts dissolving out the tooth leading to decay/cavity/ caries or a hole in the tooth.
:: IT IS BEST TO GET THE TOOTH CHECKED AT THIS STAGE ::
This comprises of a short visit to the dentist and the problem is resolved by getting a filling in the tooth which could be a silveror a white tooth coloured material known as cosmetic or composite filling. If ignored, the cavity becomes bigger, penetrating the top layer known as ename and the middle layer known as dentine getting access to the nerve of the tooth rendering it unsuitable for filling.