In practice, this works exactly as it does on Discord on desktop, allowing you to share whatever’s on your screen with other members of the server.
On mobile, this will work not only in games, but anything else you do on your phone as well. The only exception, of course, is any app that specifically blocks screen saving, like Netflix.
Discord says the feature is starting to roll out now to all smartphone users, so it might not appear on your device right away.
Meanwhile, tablet support is coming at some point in the future, says Discord.