When users sign up for your subscription plans, they might ignore the payment for some reasons (for example, close the browser without making payment after being redirected to Paypal...). In these cases, the subscription records will still remain in the system with Pending status
If you want to clean up these records like that (it won't affect the function of the system), you can use System - Membership Pro Cleaner plugin.
This plugin doesn't have any parameter, so you just need to go to Extensions => Plugins, find and publish this plugin to use this feature.