Numpy library working out cosine.

[6pathsMadara]

import numpy as np

a = np.array([1, 2])
b = np.array([2, 1])

dot = 0

cosangle = a.dot(b) / (np.linalg.norm(a) * np.linalg.norm(b))
print(cosangle)

So I'm following a tutorial and the guy doesn't explain allot in terms of why this actually gives
the value of the cosine. Please Help.