* Backup Lights: This will be a fuse specifically for the backup lights.
* Trailer Tow: This may be one or more fuses for the trailer wiring harness, including tail lights, running lights, and turn signals. A blown fuse here *could* affect the backup lights if the backup circuit shares a component within the trailer wiring.
Without your owner's manual, it's impossible to tell you the exact fuse number. Look in the fuse box under the hood (usually near the battery) and inside the cab (often under the dash). The fuse box covers will have a diagram showing which fuse controls what. If the diagram is unclear, try to find a downloadable PDF of the manual online by searching "[your truck's VIN] owner's manual". The VIN is located on your vehicle's dashboard and driver's side door jam.