HERE ARE SOME OF THE ANSWEERS BELOW:

Trevors-Axiom- said:
Looks like an in ground dog poo compost bin but I can’t find one that matches. Does the top come off?
MoreTatersPlz said :
Depending where it is in your yard and where your water line comes in/sewer line goes out, I’d guess it’s either a sewer cleanout or a shutoff valve from the main city water line (assuming you don’t have a private well). We had the shutoff line in the front yard and the sewer cleanout in the backyard at our last home, so it just depends where your utilities run.

saltyhumor said:
This must have something to do with water. Rain water, fresh water, waste water, not sure. Are you on a well and septic or city water/sewer?
How close to the house is it? Could it have something to do with sump pump? are you on a slab, crawlspace, basement?

anonymous22006 said:
My guess is an old chain link fence post that rusted off at the ground.
Instead of digging the post out, someone simply put a post cap on it.

silly_rabbi said :
It looks like the cap for a vent/access pipe that leads down to a drainage from the house. My mom has one in her yard (but hers is in the front).
Occasionally when there was a drainage issue, the workmen would pop the cap and use the access pipe underneath to get a drain snake tool into that section to clear out any blockages.

GabeLade said :
My very first thought when I saw the picture was that it was a cap for a metal post typically seen in chain link fence construction.

Meathead-12 said :
I used to see these in my neighborhood, an umbrella-style clothesline that could be stored in the garage when not used. A metal pipe was placed in the ground about a foot or so deep. The post fit inside. The cap was placed on the pipe when it was not being used.
AugustWestGarcia said:
Looks to me like it could be the breathing cap on an underground oil tank, maybe something old. And they left it there to vent the tank. You may go to the Townhall of your town and look up to see if there’s any abandon tanks on old or deeds to your house

juryjjury said:
We have similar ones in our yard. For us they are sprinkler caps. When pressurized the sprinkler head pushes the cap up and sprinkles. It is hard to lift them up to view the head underneath when not pressurized.

I forgot to mention. We have a number of them in our yard separated by umm 15-20 feet or so. But after 20 years of non use most of them are overgrown with grass or covered with dirt so only a few are now visable.

markzip said:
Wide enough to be a cap for a post used to hold one of the big old school satellite dishes?

Assuming you are in the USA and if, years ago, your neighborhood was sufficiently rural enough to warrant such a thing.

There were/are plenty of places which did not get broadcast TV and which were not passed by cable. Large satellite dishes (10 to 16 feet wide) required a very sturdy post. Given that it is in the back yard, is it possible that this site once had a good view of the southern sky?

BabalonNuith said:
I’ve seen these; I think it’s a gas vent or access of some kind. It’s definitely connected with utilities. I lived in another province then when I moved to Ontario I started seeing these on people’s lawns.

Cold_Marsupial_7461 said :
Could be an old survey marker. How old is your neighborhood? Some surveyors used just about anything to mark a property corner from angle iron to pipes to rebar. A fence post cap would do nicely. Sf there a small indentation on it anywhere?

PrudentPush8309 SAID :
That could be a cap covering the ground pipe for one of those old, TVRO satellite dishes from the 1980s. The dish on those were usually 2 to 4 meters/yards across and needed a 4 to 6 inch steel pipe to support them.

corpse_flour said
The water shutoff to my house from the street is about that size and shape, but it has a bronze top with a number etched in it. Perhaps this is something similar, but just with some kind of cover to protect it from the elements? I know they call look very different from city to city.

GreyHoundRunner said :
My first guest is going to be the cap that covers the valves for buried propane tank if you have one of those or if the house was originally set up with kerosene for heating
Bandit_the_dog99 said
I have something similar my pest control company put all the way around my house. I believe they are termite bait stations.

Capable_Can3700 said :
It looks like a termite bait station. If there is one you have more. Cover comes off to leave bait so exterminator can treat home.

PavorNocturnus13 said :
Underground oil tank for a furnace that is or was in your house.

anonymous22006 said
My guess is an old chain link fence post that rusted off at the ground.

Instead of digging the post out, someone simply put a post cap on it.

PavorNocturnus13 said
Underground oil tank for a furnace that is or was in your house.