A DMR talkgroup is simply a way of grouping many Radio IDs into a single digital contact. Or put another way, a talkgroup is a method of organizing radio traffic specific to the DMR users that all want to hear the same thing and not be bothered by other radio traffic on a DMR network that they are not interested in hearing.
Talkgroups can exist for many purposes. You can have talkgroups for countries, states, counties, regions, cities, special interest groups etc. Just about any group of DMR users could have a talkgroup assigned to them if they wished to organize traffic that they can all monitor and take part in, without having to talk to each other one by one.
Talkgroups are also specific to individual DMR networks, BUT they all generally follow the same numbering scheme. So you need to make sure that you know what the various talkgroups are for each of the DMR networks that you may use.
For example, Talkgroup 3129 on the MARC network might not be the same as Talkgroup 3129 on the Brandmeister network.
Be sure to manage your Talkgroups separately if you use more that one DMR network!