you swipe your credit card, pick up food or drink in a little monitored area and walk out with your items.
The only time I used one of those was at a conference center and I was trying to get a cheap lunch. The item I wanted was out of stock in the computer system so it wouldn't let me purchase it and the item I grabbed was charged as something else entirely (netting me a $3 discount at least)
Vending machines have been around for years and their faultiness has long been a trope in media. I'm not holding my breath for employee-less checkout