Monthly credits are the credits that are included in the monthly subscription.
For example, ProXL + Credit plan includes everything a ProXL plan offers plus 500 credits. This is the Monthly Credits. The usage of monthly credits is the same as Stock Sync credits. You can use it to add/remove products. And it will renew every month.
If you are running low on Monthly Credits and need more, you can always purchase the Stock Sync credits as a backup.