Flint is a kind of sedimentary rock which occurs naturally in many places in China, West Asia, North Africa, and Europe. Flint is easier to knapp (to shape) than a lot of other kinds of stone, and it will hold a sharp edge longer, so most of the tools that were made in the Stone Age were made out of flint. It's not as good as obsidian, but it was easier to get.
There are two ways of knapping flint. The older way was to take a rock and knock off bits until you had a sharp edge, and use that as a tool. That was easy and quick to do, but it took a lot of flint for each tool.
Here's a video of some young men knapping flint.
People who had trouble getting enough flint for everyone thought of
a better way: take a rock and knock off bigger flakes, and make each
one of those flakes have a sharp edge. Then you can get twenty or more
tools out of the same rock that made only one tool before.
If you can get flint, you can try knapping it yourself. It's not hard to make the first kind of tool, but it is a lot harder to make the second kind.