n. pl. Money, especially paper currency; bank notes; American dollars, i.e. Federal Reserve Notes (any denomination), etc.
Big or large heads indicates a larger denomination.
A lemonhead refers to a known counterfeit.
The word refers to the common appearance of a famous statesman's head (e.g. George Washington on the one-dollar bill, Benjamin Franklin on the hundred-dollar bill, etc.) illustrated on the front face of the note.
I keep a bunch of heads in my wallet. You could call me Perseus.