VNAV (Vertical NAVigation) is an automated feature of the FMS (Flight Management System) that dynamically calculates the optimum climb, cruise, and descent speeds and altitudes.
In this context, VNAV calculates a TOD (Top of Descent), taking into account all preprogrammed altitude and speed restrictions, so as to remain at cruise altitude until an idle descent at optimum speed can be made to the most restrictive point (soonest adverse altitude or speed restriction). Any unplanned speed restriction will require more fuel.