What Am I looking at?

Hello all,

The city came out because my water consumption was super high the last two weeks. I checked the crawl space for leaks then my house parimeter and while walking around I heard rushing water. I opened this up and it was full of water and more was coming. I reached in and cranked the green handle until it stopped and then it drained.

My questions:

What is this?

what did i turn off?

How did it turn on?