Once orthodontic treatment is finished, you must wear a retainer to hold the teeth in their new position. The bone and the structures around each tooth need time to adapt to the corrected position. The stability of the result also depends on how the teeth meet, as well as on factors such as jaw growth and the movements of the soft tissues (cheeks, lips, tongue).
How long the retention period lasts depends on the patient's age, the type of treatment and the original problems that were corrected. Depending on these factors, the orthodontist may recommend a fixed or a removable retainer.
This answer is for general information. Your own case needs an examination by an orthodontist.