A community within Train is like creating your own mini social media feed for a group of people at your gym or that you coach remotely that are assigned to a particular workout program.
This will help bring people in your community together, help them get better results, and increase retention.
Ideas for communities:
Masters program - you could create a community for everyone 50+ at your gym.
Barbell club - then everyone that is on the barbell program would be able to post, share, comment, and like within that feed.
Etc. You are only limited by the program types that you offer at your gym.
You can learn how to set one up here.