Limited use subscriptions give customers a set number of credits to use each billing cycle. Sometimes customers aren't able to use all of them before the cycle is up.
Enabling roll over credits allows customers to keep what credits they didn't use from the previous billing cycle.
When creating a limited use subscription you'll see an option to add the credits a customer will receive each billing cycle. After this is set, a new menu item will appear a few sections down called 'Enable
After this is figure is set, a new section will appear further down called Usage limit credits rollover. If enabled, you'll be presented with two options:
Add credits to next usage limit interval
Add credits to customer account
The former adds the unused credits to the next usage limit interval before they expire on the following interval. The latter adds them the customer's account and allows you to control how long they will last before expiring.