Subgrouping (sometimes known as "Grouping" or "Audio Grouping") is a way to collect related signals together to apply some common treatment, before sending them on to the main mix. One standard application is to group several tracks belonging to the same instrument or section (such as a drumkit or horn section), to be able to adjust their volume with a single fader, after their inner balance has been set using the track faders.
Ardour also provides VCAs that is a very flexible way to adjust the volume of a group of tracks/busses when no additionnal processing is needed.
Create a subgroup from an existing Track/Bus group is done by right-clicking on the relevant group tab, and choosing Add new subgroup bus. A new bus will be created and every member of the track group will have its outputs disconnected from other destinations and then connected to the new bus inputs. The bus outputs will feed the master bus unless manual connections have been selected in the session preferences. The bus will be named after the track group name.
Alternatively, a group can be created manually, by first adding a new bus, then, for each track to be fed in the subgroup bus, disconnecting its outputs from the master and connecting it to the inputs of the subgroup bus instead. This can be done in the global audio patchbay or on a track by track basis via the output button of each track's channel strip.
Remove a subgroup (bus) is done by right -clicking on the track group tab, and selecting Remove subgroup bus. Simply deleting the bus itself will not restore signal routing to the way it was before the addition of the subgroup bus—tracks that had been subgrouped will be left with their main outputs disconnected.