A drop-in is a casual, unannounced visit where someone stops by without planning ahead. When your friend's mom says “feel free to drop in anytime,” she means you don't need to call first or make an appointment. You can just show up at the door and knock.

The phrase works in other contexts too. At some recreational facilities, you'll find drop-in activities like basketball games or art classes where you don't need to register in advance. You simply drop in when it's convenient and join whatever's happening. A community center might offer drop-in hours for its game room, meaning kids can come and go as they please during those times.

In skateboarding and similar sports, dropping in means something completely different: it's the moment when a skater tips their board over the edge of a ramp and commits to riding down. The skater stands at the top, positions the tail of their board on the edge, then leans forward to drop in. It takes courage because once you commit, you're going down whether you're ready or not.