Occasionally means happening now and then, but not often or regularly. If you occasionally eat dessert, you might have it once a week or when there's a special celebration, not every single day. If it occasionally rains in the summer where you live, that means some summer days are rainy, but most are dry.

The word sits between “rarely” (almost never) and “frequently” (quite often). Think of it this way: you rarely see a shooting star, you occasionally visit your grandparents who live far away, and you frequently brush your teeth. Occasionally suggests something happens just enough that you notice it, but not so much that you expect it.

You might occasionally stay up past your bedtime, occasionally forget your homework, or occasionally win at chess against your older sibling. The word works perfectly when you want to say something happens sometimes, without pinning down exactly how often. It's the verbal equivalent of a shrug that says “yeah, it happens, just not all the time.”