An interprocess commnication system. D-Bus can provide a system bus and/or a session bus, the latter being specific to a user session.

To provide a system bus, you should enable the dbus service. This might require a system reboot to work properly.

To provide a session bus, you can start a given program (usually a window manager or interactive shell) with dbus-run-session(1). Most desktop environments, if launched through an adequate display manager, will launch a D-Bus session themselves.

Basically, enable the service and launch sway like this:
dbus-run-session -- sway

Notifications broken?

For some reason, I was getting an error when trying to run notify-send or whatever to launch dunst. It seems as though dbus doesn' think the DISPLAY variable is set? Despite me being able to print it out in any terminal?

Running this command seems to fix it:

dbus-update-activation-environment DISPLAY