what is sin

in math it means The sine (abbreviated "sin") and cosine ("cos") are the two most prominent trigonometric functions. All other trig functions can be expressed in terms of them. In fact, the sine and cosine functions are closely related and can be expressed in terms of each other.

When you first learn about trigonometry it will used to find the side lengths and angles in right angled triangles.  

The sine ratio of one of the acute angles it it opposite side length / hypotenuse length.

$$Sin(x)=\dfrac{opposite}{hypotenuse}$$