You won't be leaving your luggage at a kiosk. When you go to the airport, you'll be selecting bag drop on a kiosk monitor (at or near the counter) by entering your reservation or frequent flyer number or by scanning the bar code on you boarding pass. A printer behind the counter will spit out the bag tags. A UA employee or contractor will place the tag on your luggage (and usually check your ID and boarding pass).

As the above poster suggested, some kiosks are not at the counter (eg
Las Vegas), but the tags will print out behind the counter and you'll take the bags to the counter after using the kiosk. Also, Las Vegas has some multi-airline check-in kiosks prior to entering the specific airline check-in areas - these cannot be used for checking in luggage.