What I gather from Discord is that it's like a stripped down basic version of Skype, and you can maybe tie it with your Steam account so it tells you what vidya you're playing. It's primarily geared towards gamers who want a third-party voice chat platform without using in-game voice communication or Steam's often-times described "poor" voice features.

It also doesn't hog CPU use so you can play video-games with it on without murdering your RAM. You might maybe use it while playing Verdun (where player-to-player chat is restricted to text in the spirit of the old-school comp shooters it's based off of) or DayZ which'll demand a lot of RAM and where the voice-chat is restricted to the local area because like in the spirit of ARMA that sort of thing is all supposed to be a simulator of real-life, so you talk very locally or use radios to shout at people across the map.