Water irrigation system?

I hope this is not a stupid question.

We just bought a house and although the owner walked us through everything in depth she did not got over these caps located all over our yard (there are five). The lids are green and say “irrigation control box”. They are encased in a white PVC type structure. Under the can a white knob(?) is in there and appears to be able to be turn manually but since we don’t know what it is I’m hesitant to give it a go until a discuss with someone.

There are no sprinkler heads to speak of... we are on a well, septic and propane. We have been unable to locate a “main” control box in our garage our outside our house.

Any help would be appreciated l!!