Drop by means to visit someone briefly and informally, often without planning ahead. When your neighbor drops by to return a borrowed tool, they stop at your house for just a few minutes. When a friend drops by after school, they're making a quick, casual visit rather than coming over for a planned get-together.

The phrase suggests something spontaneous and relaxed. You might drop by the library to pick up a book, or drop by your grandmother's house to say hello on your way home. The visit is short enough that it doesn't require an invitation or special preparation. Someone might say “Feel free to drop by anytime” to show they welcome casual visits.

People also use drop by for quick stops at places, not just to see people. You could drop by the store for milk or drop by the coach's office to ask a question. The key idea is that you're not staying long: you're stopping briefly on your way to somewhere else, or making a quick visit that doesn't need elaborate planning.