A currency strap is a simple paper device designed to hold a specific denomination and number of banknotes. For Danbooru's purposes, this tag refers to the orderly bundle of banknotes itself. These are commonly seen carried en masse in a briefcase and can be seen in illicit business dealings and the like, as well as in ordinary transactions at banks (often in the bank vaults themselves) and financial institutions.
This tag implicates banknote (learn more).