An optional feature of the USB-C specification that allows a USB-C connection to carry signals for a completely different connection specification, that would normally require a different cable and connectors.
Specifications that can be used with USB-C Alt. Mode include: DisplayPort, HDMI, and Thunderbolt.
It's common for higher-end phones to support DisplayPort and/or HDMI over USB-C Alt. Mode as a way to physically connect the phone to an external television or monitor.
Making such a connection typically requires a special adaptor cable that has a USB-C connector on one end (to connect to the phone) and an HDMI, DisplayPort, or Mini DisplayPort connector at the other end. The adaptor cable has small circuitry in it that communicates with the USB-C device (a phone, in this example) to initiate Alt. Mode.