To salvage something is to save it...before it's too late. You might try to salvage your damaged reputation by defending yourself, or salvage a burnt piece of toast by scraping off the black residue.
As a noun salvage is the act of rescuing stuff from a disaster like a shipwreck or fire — or the rescued goods themselves. As a verb, salvage means to collect or rescue that sort of item, or more generally to save something from harm or ruin. If you want to salvage your grade, you need to stop gaming so much and start studying more.