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COS and SIN are both trigonometric functions.
cos and sin are only symbols. the correct term is actually cosine and sine.

In a right-angled triangle, there are three sides. the longest side is called the hypotenuse.
say angle A is a right angle and angle B is x.
the side opposite to the hypotenuse is called the opposite side (the side betwee angle A and B) and the remaining side is the adjacent side.

cos x equals to adjacent side divided by hypotenuse.
e.g. hypotenuse=5cm, adjacent side=4cm then cos x= 4/5

sin x equals to opposite side divided by hypotenuse.
e.g. hypotenuse=5cm, opposite side=3cm then sin x= 3/5

*sin and cos functions can ONLY be applied in a right-angled triangle.

using these functions, we can find lengths and angles of triangles.
even in non-right-angled triangles, we can apply two formulae called cosine formula and sine formula.. but the principle is quite different.
sine formula= A/sina = B/sinb = C/sinc where A,B,C are sides of a triangle and a, b, c are their opposite angles respectively.
cosine formula: B^2= (A^2)(C^2)-2ACcosb where A,B,C are sides of a triangle and a,b,c are their opposite angles respectively.

there are still a lot more about cosine and sine functions, for example the tangent function. these are only some basic information.