A training group is a group of people assigned to attend a specific training session. This particular group of people could belong to a number of different teams within a club.
For example, a training group set up named 'Senior Training' could be made up of just the senior teams at a sports club. For example the 1st team, 2nd team, and 3rd team.
You can, however, set up training groups for a single team as well. Think of training groups as a useful way to group together teams at a club.