To come by something means to obtain or acquire it, often with an implication about how easy or difficult that was. When you come by a rare baseball card, you might have found it at a yard sale or traded with a friend. When a scientist comes by an important discovery, she might have worked years to find it.

The phrase often appears when someone wants to emphasize that getting something wasn't easy: “Good friends are hard to come by” means true friendship is valuable and rare. If honest advice is hard to come by, it means you can't find it easily. On the other hand, if apples are easy to come by in autumn, they're plentiful and simple to get.

The phrase can also mean to visit someone casually. If your grandmother says “Come by anytime,” she's inviting you to stop in for a visit whenever you want. You might come by a friend's house after school or come by the library to return a book.

Notice how the two meanings connect: in both cases, you're arriving at or reaching something, whether it's a place or a possession.