Some gaming subreddits replaced links to IRC with Discord links after Discord's 2015 release. IRC, or Internet Relay Chat, was the older protocol many online communities used for real-time text discussion.
Discord's appeal was that it packaged text channels, voice chat, persistent communities, moderation tools, and invitation-based access into a friendlier modern service. Users did not need to configure a separate IRC client or learn commands to join a community. Discord servers also made it easier for gaming groups to organize channels around games, teams, events, and off-topic conversation.
The wording refers to a trend among some gaming subreddits, not a universal migration. IRC remained active and continued to serve open-source, technical, and other communities. Discord also did not replace IRC at the protocol level: it is a proprietary communications platform with its own infrastructure. After launching publicly in May 2015, Discord spread through gaming communities, esports, LAN tournaments, Twitch relationships, and subreddits devoted to games such as Diablo and World of Warcraft.