The tab "Credits" in your System Settings is only useful when you use credits in your system.
In your gym or personal training sessions, you might offer different classes, or services. If these services are charged differently, or you want to set them apart, you can fill these in as a credit in the System Settings. For example: if you offer group sessions, personal training sessions and general entry, you can create credits for each of those.
You can choose whether people can have negative credits and when the credits will be deducted. Read more about that here.
After you have added the services, you will be able to see them in 3 different places: when creating a membership, when creating a point of sale product, and when adding costs to your scheduled activity. In all those places you can choose to attach credits of that service to your membership/product/scheduled activity.