Other forms: refilled; refills
To refill is to fill something again, the way you refill your motorcycle's gas tank or refill your cat's water bowl when you notice it's almost empty.
You can also refill a prescription, by going to the pharmacy and getting another month's worth of medication. The bottle of pills itself can also be called a refill. People commonly use this word as a noun for the act of filling up, too: "Hey, grab that waiter and ask for a refill on the Cokes!"