Put a mass $m$ on a planet of mass $M$ and radius $r$, then write the same force twice: $\dfrac{GMm}{r^2} = mg$. The little mass cancels off both sides — which is why a hammer and a feather land together — and what survives is $g = \dfrac{GM}{r^2}$, read aloud g equals big G M over r squared. Here $g$ is the gravitational field strength in $\mathrm{m/s^2}$ (equivalently newtons per kilogram — the same unit wearing two hats), $M$ is the mass of the world making the field in kilograms, and $r$ is the distance from that world's centre in metres — its radius, if you are standing on it.

Keep the three symbols apart, because an exam will absolutely try to blur them. Big G is the universal constant, $6.67 \times 10^{-11}$, identical everywhere. Little g is local: 9.81 here, 1.62 on the Moon, and smaller still the moment you climb. Weight, $W = mg$ — W equals m g, with $W$ a force in newtons and $m$ the object's mass in kilograms — belongs to one particular object standing in one particular field. Your mass is your business; your weight is a negotiation with a planet.