Why A trap is a parallel LC resonant circuit inserted in an antenna element. At the trap's resonant frequency it presents a very high impedance, effectively disconnecting the wire beyond it so the antenna is electrically short for that band; at lower frequencies the trap acts mostly as a loading inductance and the full length radiates. That lets one physical antenna resonate on several bands from a single feed line.
Watch out The talk of resonant circuits makes filtering answers like notching spurious frequencies tempting, but the trap is inside the radiating element to set its electrical length, not in the feed line to clean up the transmitter's output. Traps also do nothing to stop you from transmitting out of band.
Trap = tuned circuit that "traps off" the rest of the wire, so one antenna covers many bands.