There are 2 bins in the machine - one is for the cleaning solution (you have to buy a special one for it) and water. The other is for the dirty water. It picks up dirt and stuff, too, so you aren't supposed to have to sweep before you use it. I generally use it in my laundry/mud room, which has tons of pet hair, so I do a quick Swiffer (or vacuum) first. Then you set up your virtual walls if you need to and push the 'clean' button. It does it's thing - takes a couple of hours - then lets you know when it's done, so you can empty and rinse the dirty water bin.
I only plug in the charger when I know I'm going to be using it, because if you leave it plugged in the battery dies pretty quickly, and they're expensive to replace. I learned that lesson the hard way!