If memory serves me right, the VFR advisory frequency is the frequency that VFR traffic should contact to transit the airspace (or for flight following) when it is not the same as the primary control tower frequency (Generally it is a 24 hour approach frequency at an airport where the tower closes at night, hence no full time ATIS).
Although a few airports have such frequencies published in the A/FD (CYYZ, KNID are examples), I can't find such references on the actual sectionals.