Talk groups are used in trunked radio systems to identify groups of users sharing the pool of frequencies in the system. They do not correspond to discrete frequencies because the trunking system assigns frequencies independent of the user. For example, a municipal trunked system would have talk groups for police, fire and other city services. All the users would share the pool of frequencies, but each would have their own talk group and not be aware of other users. This is why programing trunked systems can become so complicated. Once a system is programmed you can lock out talk groups that you're not interested in. Most scanners can also operate in a search mode to find new talk groups.
As you are asking about trunking, there are 2 links (always blue) in that third bullet on this topic. In addition the Easier to Read manual links are at the bottom. Take your time and read - there's a lot of new concepts, very different from the things we hams use, and it sometimes takes time
When you're ready I have a AA county FreeScan file (I used to live in GB) that I can send you as an example. Contact me at ka3jjz (at) comcast (dot) net when you're all set up. Make sure you tell me *precisely* what it is you're looking for