It's probably a cleanout to the septic system. It is cracked and no longer sealed, but there's probably not much of anything going in. If it doesn't smell, you might just leave it.
To repair it, you need a new threaded fitting. Hassle is, when PVC is attached, it is solvent welded - the two pieces become one part (not the cap, that can be removed - it isn't glued in).
You might be able to saw through the fitting in a few places, then peel it off. It will likely take some of the pipe with it, but maybe not enough to ruin it so you can't glue on a new one. If it does, then you'd need to cut it off, add a coupling, then a short stub of pipe and a new fitting.
While annoying, you could raise it so you wouldn't be likely to chip it with the lawn mower. If you put a threaded fitting on the pipe, you could probably find a brass cap and cleanout that might be a little more robust, or just put a fake stone or some other thing around it. A sprinkler burried control box might be a good idea, too. They come in green, and wouldn't stick out as much. If you cut the existing fitting off, then installed the box, you could make it essentially flush, rather than sticking up, too.