At the completion of the active phase of orthodontic treatment, braces are removed and removable appliances called retainers are placed. To retain means to hold.
Teeth must be retained or held in their new positions while the tissues, meaning the bone, elastic membranes around the roots, the gums, tongue and lips have adapted themselves to the new tooth positions. Teeth can move if they are not retained. It is extremely important to wear your retainers as directed!