A one-hitter is a small pipe that people use to smoke herbs. It's called a one-hitter because it's designed to hold just enough herb for one hit or puff.
Imagine you have a toy that you can fill up with some small candies. You can only put in one candy at a time and when you want to eat it, you take one candy out and eat it. That's kind of how a one-hitter works, but instead of candy, you fill it up with herbs and instead of eating it, you smoke it.
To use a one-hitter, you put a small amount of herbs into the bowl at the end of the pipe. Then, you place the other end of the pipe into your mouth and light the herbs with a flame. When you inhale, the smoke from the burning herbs goes into your mouth and then into your lungs.
One-hitters are often used by people who want to smoke discreetly because they're small and easy to hide. They're also a good option for people who don't want to smoke a lot of herb all at once.