From my understanding, if you see a game where Team A is -15 and the other team is +15, that line will often attract the public to bet on the +15, but in reality, the -15 might end up covering easily. Another example is when you see a strong team favored by just -2 against a struggling opponent, that small line could be a trap.
Basically, the idea of a trap line is when a spread looks too good to be true. When the line doesn’t match what you’d expect based on team performance, it’s a warning sign that something else is at play.