Every house should have at least one plumbing clean out port.
I don't know when they began putting these into houses. It seems to me in the 30s or 40s.
It allows access into the plumbing drain line to clear out a clog in the line inside the house or between the house and sanitary sewer in the street.
In recent decades access ports began to be installed in the yard. This convenience intended that cleaning out the drain might not require a plumber to come into the house and expose the house to sewer gases.
That was a good idea.
Current codes require the ports to be installed indoors and out. The IRC Section 708 has all the definitions regarding that. And there are many!
Coming to a house recently this is what I saw.
The plumbing clean out in the yard was broken, exposing the interior.
I see this from time to time.
Usually it's the plastic drain cap that someone has run over with a lawn mower.
Comparing these two photos, the one on the left is in my yard.
I replaced the cap on mine by inverting it so the knob does not protrude upward.
That way I can mow over it and not worry about breaking and opening it.
But the clean out port in the yard should not be open.
Debris or animals could get inside and clog the drain.
If the cap gets damaged or the port opened it should be repaired. Sooner is better than later.
My recommendation: home ownership requires that even the little things be paid attention to. As to these ports, when they are near the sidewalk and children pass by these holes can become an attractive nuisance whereby dirt, stones, small animals, or even GI Joe or a Transformer could be introduced and clog the line! So keep it sealed!
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
I did not know that I have a plumbing clean out - I will have to look for it!
If you take off the cap you can gain the full benefit of the experience Lise.
I bet the balance is 50 - 50, broken ones to good ones.
A clean out port is better than having to pay for someone to come out and dig up the backyard. We recently had the County come out and inspect the sewer lines in the neighborhood. Several homeowners had "roots in stack" that required an expensive repair. Yet still cheaper than a basement backup!
My neighbor called to complain that the plumber charged $600 to snake his drain line Fred. When I looked in the bushes in the front yard I saw an wide-open pipe. That's the reason! What's the cap cost???
Stephen - sure! See comment above! I don't know what 'roots in stack' are, but likely not like pancakes.
Mine is right outside just off of my front porch. I know exactly where it's at! Thankfully I've never had to use it but it's there if I need it.
It's good to know where it is Nicole. And you probably will never use it, but keep it covered!
Jay Markanich
I had one when I looked down I saw rag and a ball in it.
Yummie! Did you play flag tennis ball?
I have had animals look up at me Don!