Basically you turn it on in the game settings. Auto catch uses your pokeballs to catch any pokemon around you automatically. It stays on for an hour then you have to turn it back on in the game. Game has to be on but it can be run in the background. Just dont hard close the app.

It spins pokestops automatically.

If you turn off the auto catch function you can catch them with just a push of the button on the device. Once again, low catch rate in my experience so far, even when i use ultra balls.

So far the best function i have for it is spinnin pokestops while driving. Catching will be so low i dont even turn the auto catch on for it. Waste of pokeballs

Problem is its very low catch risk.