Source: Flickr
Why Do The Money Changers Chop Marks?
Verifiers stamp chop marks on a bill after it passes the ordinary security checks. Someone marks the chit with a specific sign to indicate provenance. This process:

Verifies to future possessor the legitimacy of the bill.
Monitors its movement along local financial chains
Builds trust where there is no strong central verification
What Do These Marks Appear As?
Appearance of Chop marks:

Arrows, stars or birds and other symbols
Stylized letters or initials
Special symbols such as one of a bow and arrow, as on a $20 bill recently

Source: Flickr
They use ink — blue, black, or red — to prevent obscuring the official security elements.

Are Chop Marks Legal?
Federal statutes in the United States outlaw the mutilation of currency to the point of affecting its value as legal tender, but chop marks do not rise to that level. These tiny stamps have no impact on the functioning or value of a bill. But machines and some banks may refuse heavily defaced bills because of appearance.

A Quiet Story in Your Wallet
A marked bill might have run through markets, banks or booths so some distant land. They’re all stories about verification and movement and trust across borders. When you receive one, you’re holding a slice of that global expedition.