It basically means that the mint/date is rare and that makes it "key" or hard to acquire. Many collectors will talk about the series that they are collecting like Buffalo Nickels, Mercury Dimes, Walking Liberty Halves and how they need the key dates to complete their series. Key dates are very expensive indeed. Look up the 1916-D Mercury dime, a key date that will bring hundreds of dollars in AG (about good) condition.
1909-S VDB another example, I found two metal detecting. One was in perfect condition at a school playground(kid took it to show n tell prob??) The other was really green from a construction site....
