A QR code, or Quick Response Code, is a two-dimensional machine-readable barcode that can be used to represent an address. The use of a QR code makes it easy to scan an address, rather than having to type or copy it.
The Trezor Wallet shows receiving wallet addresses as QR codes, so they can be easily scanned from a mobile device or using another camera. If given access to a camera, the Trezor Wallet can also scan QR codes of an address to send funds to.
See also: Address (basic)