Whenever teeth are moved today we need to retain (hold) them. We need to convince them over a long period of time to stay where we placed them.
Partially, it is about insuring that old problems do not reoccur but we also don’t want new problems to arise. It is for this reason common philosophy is that a certain amount of retention is forever. Hopefully, this does not mean you have to wear your retainer every day for the rest of your life, but you do have to keep it around.
There are different kinds of retainers but the ones we use most frequently are clear retainers, Hawley retainers, and fixed retainers.