You've likely heard the term "unmarked money" in Hollywood heist films. In a typical bank robbery scene, the criminal will ask for the money in "unmarked bills." But this isn't just creative license; if you operate a retail business, you should understand the importance of unmarked and marked money.
When you mark money, you don't put a physical mark on it -- you simply record the dates and serial numbers from the money so it can be identified. The Portland Police Bureau recommends marking a series of non-consecutive $10 and $20 bills to hand over in case of a robbery, to assist the police in tracking down the criminal. Unmarked bills are those that you have not recorded and that you will use for regular transactions.