A cashier is someone who works at a store ringing up people's purchases. Most cashiers use a cash register for keeping money and making change.
Fast food restaurants, grocery stores, and book shops all hire cashiers to scan purchases or manually enter their prices into a register, process the customer's payment, and put the items in bags. In Britain, you can also call a bank teller a cashier. The word is from the Middle French caissier, "treasurer," which comes from caisse, "money box."