Can someone help me identify what this is please.? It was mounted by one of my batteries. 2009 chevy tahoe SSV

 

This looks like a Ford style starter solenoid. It could have been to power an accessory like a winch.
More likely, it is used to enhance starting ability on a GM. The positive wire from the battery attaches to one large terminal. The other large terminal connects to the large terminal on the starter.
The trick is to remove the starter trigger wire (usually purple) from the starter and attach it to the “switch” terminal on the solenoid.
Then connect the terminal where the purple wire was to the large terminal on the starter.
Now the starter receives no current until the driver cranks the engine. When it receives current the starter turns with the built in solenoid contacts engaged. This is sometimes called a “hot start” modification. I think some retailers sell a hot start kit.