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.
To create a subgroup from an existing Track/Bus group, context-click on the relevant group tab, and choose 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 you have selected manual connections for the session. The bus will be named after the track group name.
Alternatively, you can create a group manually, by first adding a new bus, then, for each track you want to feed the subgroup bus, disconnect its outputs from the master and connect it to the inputs of the subgroup bus instead. You can do this in the global audio patchbay or a track by track basis via the output button of each track's channel strip.
To remove a subgroup (bus), context-click on the track group tab, and select Remove subgroup bus. You can also simply delete the bus itself. Note that this operation 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 disconncted.