Newton's law of universal gravitation
An apple falls; the Moon does not. Isaac Newton's leap was to suspect that these are the same thing — that whatever pulls the apple down also keeps the Moon swinging around the Earth, and the Earth around the Sun. One force, reaching across empty space, holding the whole cosmos in step. That is why it is called universal: it acts between every pair of masses everywhere.
The law states that two masses M and m attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance r between them: F = G M m / r^2, where G is a tiny constant of nature (about 6.67 x 10^-11 in SI units) measured in the lab. 'Inverse square' means doubling the distance quarters the force; tripling it cuts the force to a ninth. The pull is always attractive, always along the line joining the two bodies, and acts equally on each of the pair.
This single sentence is the engine under Kepler's three laws: feed it into the mathematics of motion and out come ellipses, equal areas, and P^2 proportional to a^3, all at once. It also predicts new things Kepler never saw — how to compute a planet's or star's mass from an orbit, why tides rise twice a day, and how comets return. It reigned for two centuries until Einstein's general relativity showed it to be an extraordinarily good approximation that nonetheless breaks down in strong fields and at the highest precision.
Knowing G, the Earth–Moon distance (about 384,000 km), and the Moon's 27.3-day period, you can rearrange F = G M m / r^2 plus circular-orbit motion to weigh the Earth: out pops a mass near 6 x 10^24 kg, without ever leaving the ground.
Gravitation turns a timed orbit into a scale that weighs worlds.
Newton's law is not the final word: it cannot fully explain Mercury's perihelion shift or the bending of starlight, where general relativity takes over. It is wrong in detail yet superb in practice — accurate enough to fly spacecraft across the Solar System.