From the Super Mario Wiki
A Pitfall (known as a Pitfall Seed in the Animal Crossing series) is an item that appears in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U, bearing the same appearance as it had in the Animal Crossing series, from which it originated. Pitfalls are white balls with a single red exclamation mark on them. They are used in a similar manner as they are in the Animal Crossing games; once a pitfall is thrown on the ground, it will be buried and become completely invisible, with no change in the terrain. If a player walks over the spot in which the pitfall is hidden, a pit will open up under them and they will fall in, immobilizing them and leaving them open to attack.
Pitfalls can also be set in suspended platforms. In this case, if a player walks over the spot where the Pitfall is hidden, they will fall through the platform. However, they will not be buried if the main platform is below them. If a Pitfall is thrown at an opponent while they are in the air, the Pitfall will send them straight down. This tactic can be used on recovering opponents.
Trophy information from Super Smash Bros. Brawl