That’s the name we use to designate software like GitHub, GitLab and similar, which provide repositories hosting and tooling like issue trackers. It’s supposed to be named because of SourceForge, the oldest of such tools, although I didn’t hear the term “forge” before the last 5 years or so, long after SourceForge demise, so I imagine there is a bit of nostalgia in this name (not sure who is nostalgic of SourceForge, though 😂). The wikipedia page : en.wikipedia.org/wiki/Forge_(software)