You can pay for your food from the buffet either at the manned checkouts or at the automatic terminals.
At the terminals, a camera recognizes what is on your tray.
You then have to place your plate on the scales in both ways and then hold your student ID card up to the reader.
For the menu of the day in the canteen, you have to hold your ID card up to the reader at the turnstile before it is served. The money will then be deducted and you can take your meal at the counter.
In Cafe Botanik or at the vending machines, you can also pay with your student ID, which simply has to be held up to the reader.
How does the money get onto the card? -> How do I top up my Uni card with money?