My antenna pre-amp has an FM Trap. What is that and how does it work?

If you have a nearby FM station it, or a harmonic can bleed on other TV frequencies, causing problems with Picture and/or sound.
If not using the FM band for stereo, then switching trap on shouldn't hurt, the built in trap, traps all in the FM Band.
If you are near a strong station, and want FM, and 1 station causing problems, then you want a Tunable FM Trap.
Tune Field strength meter to offending frequency, then adjust until at minimum signal on that station.