Steam (https://www.steampowered.com) is a software content delivery system developed by Valve software (https://www.valvesoftware.com). There is some free software available, but for the most part the content delivered is non-free.
This package enables Steam to be easily installed via an appstream client on non-i386 systems as long as the i386 foreign architecture is configured. This is the default on Ubuntu, but on Debian, you may need to run as root: dpkg --add-architecture i386; apt update
This package is not needed if you only want to install Steam directly with apt or with a non-appstream client like Synaptic.