The screenshots above tell the entire story: Hudlum is a lightly customizable volume indicator in the lower center of your display, where the macOS volume indicator used to be. It comes up automatically whenever you mute, unmute, or change your system volume.
Note that Hudlum doesn’t change anything about macOS. It appears in addition to macOS's own volume indicator, but it's so much easier to see, you might not even notice that little slider in the corner anymore with Hudlum running.
Think of Hudlum as a pet peeve turned app. As explained in the release announcement, we don't think macOS 26's volume indicator works as well as the old one did. Hudlum solves that problem.