The pilot jet refers to the idle jet of the carburator... most likely, they're talking about modifying or tuning the existing idle jet to release less gas, leaning it out so that you're burning less gas while parked in traffic... the pilot jet provides all of the fuel at idle and at low throttle openings.

A good mechanic can get you maybe 1 or 2 km/l extra by tuning/changing your jets... a great mechanic will get you even better mileage and some extra power to boot... but be sure the guy knows what he's doing!

Tsikoteers, any advice on good carb tuners for this guy?