What is cos theta and sin theta?

They are mathematical functions.

Most people are introduced to them as trigonometric functions. In the context of a right angled triangle, with one of its angles being theta,

Cos(theta) = The ratio of the lengths of the adjacent side and the hypotenuse.

Sin(theta) = The ratio of the lengths of the opposite side and the hypotenuse.

More advanced mathematicians will know them simply as the following infinite series:

Cos(theta) = 1 - x2/2! + x4/4! - x6/6! + ...

and

Sin(theta) = x/1! - x3/3! + x5/5! - x7/7! + ...

n! = 1*2*3* ... *n