HDMI Alternate Mode is a DisplayPort feature that allows an HDMI source device to send audio and video signals over a USB-C connector to a DisplayPort monitor or other display device. This enables you to use a single cable to connect your HDMI source device (smartphone, laptop, tablet, etc.) to a DisplayPort monitor or other display device.
HDMI Alternate Mode also supports the USB Power Delivery specification, which allows you to power your display device using the USB-C connector on the cable. For example, you can connect your MacBook Pro with Thunderbolt 3 (USB-C) port to a monitor that uses the DisplayPort protocol and get both video and power through the one cable.