A fixed retainer is bonded to the back surface of the teeth. It is most often placed on the lower front teeth, but it can also be fitted on the upper teeth. It is bonded with composite material to the back surface of each individual tooth, and no material gets between the teeth. This makes flossing possible, but, just as when cleaning with braces, the floss cannot be passed between the teeth in the usual way with a sawing motion. Cleaning is done with floss designed for braces (with a stiff end), which is threaded under the retainer from the gum side and can clean the small triangle of gum at the base between two teeth.
This answer is for general information. Your own case needs an examination by an orthodontist.