A retainer bonded to teeth is a passive fixed device that is bonded to one more or teeth, and is used to prevent the movement of the teeth in relation to each other. This can be used once full bands are removed so to prevent the movement of teeth in an unstable area, often the lower front teeth. This often a wire bonded to the backs of upper and/or lower teeth. 

When full bands are removed, the muscles of the mouth, may find the teeth, that are now in new positions, not right and push the teeth back to where they were before.

The fixed retainers stop the teeth moving due to these muscle forces. Unfortunately, the wire used to splint the teeth together, creates a triangular space between the teeth and a raised area, (with the bond) on the teeth, for bacteria to build-up. This also hinders normal tooth brush cleaning.

Effectively the surface, where the wire has been bonded, has been split into tooth. Above and below the wire.

To clean the tooth above the wire:

Angle the tooth brush, from the biting surface down vertically, so the tooth brush bristles meet the wire and can go under the wire bridging between the teeth. Press and wipe firmly against the tooth, back towards the biting surface.