Every account in stellar must keep a bond that "pays" for the storage they use in the distributed account database. This bond is represented by an enforced min

Every account in stellar must keep a bond that "pays" for the storage they use in the distributed account database. This bond is represented by an enforced minimum balance on your account that scales with the number of sub entries for your account. Practically: Most accounts can only spend down to their last 20 lumens, the most common bond amount for a minimal account. To "redeem" the bond and spend the last 20 lumens you have to issue a special "account merge" operation that is logically distinct from the normal payment operation.