We set up a bridge between the Lemmy.world General Matrix room and the Public-1 channel on the Lemmy World discord server yesterday. It's not perfect as emoji reactions aren't visible and some minor things like how it handles edited messages but other than that it seems to be working well.
Now people on both chat clients can interact with eachother!
Two different, incompatible chat services exist - Discord and Matrix. This allows users of either service to post to a channel that has been connected on each side and messages are basically cross posted between the two, allowing users on each service to almost chat like they are all on the same channel despite being on two different chat services.