It is an estimate of the power demand on the engine during driving.
It is calculated using the second-by-second speed values in a driving schedule, along with information about the type of vehicle being operated. The VSP is used by MOVES to determine the amount of time a vehicle spends in each of the 23 operating mode bins.
For each operating mode, MOVES has an emission rate (in units of grams emitted per hour of operation) that is used to calculate the emissions emitted during driving.
You can find out more about VSP and operating modes by reading Section 4.2 and Section A.3.3 in the report:
MOVES2004 Energy and Emission Inputs (PDF)(175pp, 5.5MB, EPA-420-P-05-003, March 2005, About PDF).