That comes from the ratio of sides in a 30-60-90 triangles. If the shortest side, which is the side of the triangle opposite 30 degree angle, is x, then the hypotenuse is 2x. Because we are given that the hypotenuse is 8, the base of that triangle has a length of 4, and the side opposite the 60 degree angle is equal to $4\sqrt{3}$, which also happens to be the height/altitude of the parallelogram.

J says

Hi, could you elaborate on why the base of the triangle is 4?

GMAT Quantum says

That comes from the ratio of sides in a 30-60-90 triangles. If the shortest side, which is the side of the triangle opposite 30 degree angle, is x, then the hypotenuse is 2x. Because we are given that the hypotenuse is 8, the base of that triangle has a length of 4, and the side opposite the 60 degree angle is equal to $4\sqrt{3}$, which also happens to be the height/altitude of the parallelogram.