A Bitcoin ATM allows users to buy and sell Bitcoins for cash.
Bitcoin ATMs can be more expensive than online transactions as the infrastructure cost is higher i.e. building an actual ATM and the steps that are actually needed to transfer the money and Bitcoins.
The first step is to scan her identification which the ATM then validates.
Then the user feeds in some cash and generates a QR code from her wallet a QR code being a large square of black and white pixels. Then the user sends the Bitcoins to the presented address.
Some ATMs generate a paper wallet or offline wallet essentially a bearer instrument of the Bitcoins.