Retainers hold your teeth in their new position after braces or aligners. Without them, teeth can gradually drift back.
There are removable and fixed retainers, each with advantages. Your orthodontist will recommend the right option for you.
Wear your retainer exactly as instructed, clean it gently with lukewarm water, and store it in its case when not worn.
Retainers are the key to keeping the smile you worked for.