>>535501296
It adds a repository from which you can install Flatpak programs (presumably Dolphin) with the appropriate `flatpak install some.shit//version` command (where "some.shit//version" is the exact package name, which you can generally find with `flatpak search shit`). In this case, I assume you'll want to do `flatpak install org.DolphinEmu.dolphin-emu`.
But I don't know why you'd add a repository if your Flatpak installation already came with Flathub as a remote (which it probably did), because it looks like Flathub has the same package. If the one in Flathub is unofficial and thus less trustworthy or something, then I don't know how you would ensure that Flatpak installs it specifically from the official Dolphin repository. (I don't use Flatpak very often.)