the gravitational constant
Newton's law tells us gravity depends on the masses and the distance, but it needs one more thing: a number that says how strong gravity actually is in our universe. That number is the gravitational constant, written with a capital G. It is the same everywhere, for every pair of objects, from two marbles to two galaxies, which is why we call it a universal constant.
Its measured value is G = 6.674 x 10^-11 newton-metres-squared per kilogram-squared (N m^2 / kg^2). Plug it into F = G m1 m2 / r^2 and the units work out to a force in newtons. Because G is such a tiny number, gravity between ordinary objects is extremely feeble; it only becomes a powerful force when at least one mass is astronomically large, like a planet or star. G was first measured by Henry Cavendish in 1798 using a delicate torsion balance that could detect the faint pull between lead spheres.
Do not confuse big G with little g. Big G is the universal constant, roughly 6.674 x 10^-11, and it is the same throughout the cosmos. Little g is the gravitational field strength at a particular place, about 9.8 m/s^2 at Earth's surface, and it changes if you move to the Moon or up a mountain. Interestingly, G is the least precisely known of all the fundamental constants, because gravity is so weak that it is genuinely hard to measure in the lab.
Using g = G M / R^2 for the Earth (M = 5.97 x 10^24 kg, R = 6.37 x 10^6 m) gives about 9.8 m/s^2, matching what we measure when things fall. G is the bridge from an object's mass and size to the field strength you feel.
The same G that seems minuscule reproduces Earth's familiar 9.8 m/s^2 once you use Earth's real mass.
Big G (about 6.674 x 10^-11, universal) is not the same as little g (about 9.8 m/s^2, the local field strength that changes from place to place).