He asserted the equivalence of mass and energy, which would lead to the famous formula E=mc2
Einstein showed how to calculate Avogadro's number and the size of molecules.
He explained that light energy came in chuncks or quanta, now called 'photons'. This explanation changed the way researchers thought about the nature of light.
He discussed the Brownian motion that helped in proving the existence of molecules.
He also gave forth explanation regarding the dynamics of individual moving bodies.
And last, but not the least, he explained the nature of space and time.