With a Migros card there is a discount on certain items that you'll see clearly marked as you go around the store.
At the checkout, He/She'll probably say " Migros card var mi ? " You say "Var" and hand over the card to be swiped, and they'll return it to you.
You'll get the discount taken off when she/he has finished scanning your goods.
You also get points added to your card and can get discount vouchers and/or a "Migros Cheque" from the machine at the entrance. See one of the staff at the desk, or, the Security Guards are quite helpful too.
Simpler than Tesco's or Sainsbury's really !