There is this thing outside near the side of my house in the back and it looks like a plumbing pipe coming?

out of the ground with a cap on it. The cap is missing a chunk of it but it still screws on but when it rains water and stuff can get in it. What is this thing, and does it matter that it's broken, and if so, where do I get a new cap for it and what is the cap called? Thanks.