The pitfall item has been in the series since Brawl. It comes from Animal Crossing. In Animal Crossing games it's a joke item. You bury it in the ground and if someone walks over it they get stuck for a moment.
In Smash Bros. it's a throwing item.
And it's pretty easy to miss with it.
But if you get an opponent stuck, if they have a high damage tally, it's an easy strong smash attack and they're KOed for good. While not my favorite item, it's helped me out a good number of times.