reserves is the money that a bank holds aside just in case they run out, they'll have money to back them up.
When a bank runs out of reserves they can either get loans from the government or file bankruptcy.

required reserves is 25,000. the bank has excess reserves of 75,000, they can loan out everything but the required reserves

so assuming they have no loans, they can loan up to 475,000.

reserving bank

When you borrow money from a bank they pull cash from the bank's reserves. This collection of cash is the net cash reserves within the bank or its network from depositors in the system.