It was an excise selectively imposed on growers of certain grains which had been converted to distilled spirits in order to ease transportation and other such concerns at the time. The rebellion was about the SELECTIVE nature of the imposition in that it taxed growers of this type of commodity while growers of other commodities went untaxed.
I thought it was an excise paid by the distiller, who was often the grower too. Small distillers were required to pay a tax per gallon while large operations paid a lower flat rate.
The point is that the tax was an excise tax, not an income tax.