G, or the “gravitational constant”, controls how strong gravity is in our universe. It is most commonly used in this equation:

F = G * m1 * m2 / r

Where m1 and m2 are two masses and r is the distance. It applies between any two masses. For those of us on earth, we mostly feel this force attracting our bodies to the planet, but if you put two bowling balls close to each other, they exert a very small gravitational force on each other that can be measured.

The interesting thing is G itself. As this video points out, if it were bigger or smaller by more than a percent or two, our universe would be completely different: