Can someone tell me an Summary of keplers 3rd law?

If you square the 'year' of each planet, and divide it by the cube of its distance to the Sun, you get the same number, for all planets

It means that if you know the period of a planet's orbit (P = how long it takes the planet to go around the Sun), then you can determine that planet's distance from the Sun (a = the semimajor axis of the planet's orbit).