Although there are some very small changes to your teeth that can be made with removable appliances/retainers, retainers primarily 'retain' or hold your teeth in their current position. After your orthodontic treatment is finished and your braces are removed, you will need retainers to hold your teeth in their new positions.
The retainers are individually designed to prevent teeth from reverting to their original positions. Retainers can be a removable appliance or a fixed wire bonded behind your front teeth.
Generally, retainers are used full-time for the first 6 months to a year; after that, the retainers are worn at night. Fixed retainers which are mainly bonded on the inside surfaces of lower front teeth are normally kept in place for as long a patient can manage hygiene and cleanliness.