The gravitational field strength of a planet measures how much a 1kg mass weighs on the surface of a planet. It is measured in Newtons per Kg (N/Kg)
Weight = Mass * Gravitational Field Strength
N = kg * N/kg
E.g. How much did Neil Armstrong weigh on the moon?
Mass of Neil = 80kg
Moon GFS = 1.6 N/Kg
W = M*GFS = 80*1.6 = 128N = 12.8kg