'Credit from past subscription' is marked next to a customer's payment, when an upgrade/downgrade has been made and credit has been a carried across from a previous payment.
The customer payment tab can be found under Customers >> Customer List >> Select customer name >> switch to payment tab.
For more information on the payment and credit, click 'Full Details'
You will now be able to see the total credit that was applied from a past membership, along with various other information on the transaction.
For more information on upgrades/downgrades and why credit is sometimes applied, see:Upgrade or downgrade a membership on behalf of a customer