When you pay a quick, casual visit to someone, you drop in. Is your grandmother under the weather? You may want to drop in to give her a hug and deliver some homemade soup.
You might drop in on your neighbors on your way home from school to say hi (and see if they've been baking their famous chocolate chip cookies), or you might drop in at the library on your way home from school. Whenever you drop in, you're stopping by briefly and usually without planning ahead. You could also say drop by, stop in, or come by.