A Bitcoin ATM (Automated Teller Machine) is functionally similar to the ATMs commonly used to deposit cash into a bank account. A regular ATM will take your cash and update your bank balance with an IOU that you can use to pay others.
With a Bitcoin ATM, your cash is exchanged for Bitcoin, which is deposited directly into your own Bitcoin wallet. It is also often possible to purchase selected altcoins at Bitcoin ATMs, and some outlets will also include the option to sell your crypto for cash!