A replacement is the thing that fills in for something that's missing, or the act of substituting for the missing thing. The replacement for your absent teacher is an annoying substitute. Sigh.
When kids lose their baby teeth, replacements soon grow in their places. And when kids need to have their teeth cleaned but their dentist is on vacation, her replacement will be in charge of taking x-rays and checking for cavities. Replacement comes from the verb replace, which adds the "back" prefix re- to place, "to put somewhere."