What is this plumbing fitting?
Bayonet style sprinkler stand pipe with a pipe nipple, 90 degree fitting and MHT hose adapter. Lower section similar to this part from Rainbird.

Normal operation is that you lift the cap, push the sprinkler standpipe with attached impact sprinkler in and give it a quarter turn to lock it into the socket. They've adapted the operation to put the water into standard lawn hose in this instance.