The actual title of the Stamp Act is the far less descriptive and far drier: "Duties in American Colonies Act 1765."

Wow. That's a real catchy title.

"Duties" refers to a payment here, which is kind of an archaic usage but you still see it from time to time. Specifically "Duty-Free" shops in airports are places where you can get goods without paying taxes—or duties—on them. Think of it as another word for tax.

The title of the act refers to the new taxes the British crown will be collecting in the colonies. That's it.

It generally gets shortened to the Stamp Act because it's more distinctive and quicker to say. It was all about those stamps, after all. It could probably be called the Paper Act, but we've already collectively named it: the Stamp Act it is.