A retainer is a piece of plastic and metal that is custom-made for each individual kid who needs one. It fits the top of the teeth and mouth. No two retainers are alike, even though many look similar. Retainers are really common. In fact, most people (kids and adults) who have braces have to wear a retainer for at least a little while after getting their braces taken off. Other people wear them to close gaps in their teeth, to help with speech problems, or to solve certain medical problems.