The parameter for the sin function is an angle. The angle can be specified in either radians or degrees, depending on the context. The output of the sin function is the ratio between the opposite side and the hypotenuse on a right triangle like this.
There are many different algorithms which can be used to calculate sin(a), but all that I know require a decent understanding of Calculus (Taylor Series) or Trigonometry (CORDIC). You can find a Python implementation of the CORDIC algorithm here.
In a more general sense, the “operation” of the sin function is the “sin” operation. In maths, one very rarely goes lower than f(a) = sin(a) and when one does, it’s often just to find an alternate way to estimate the value of sin. One can think of the sin function as being similar to ln(x) in that it is much closer to an operation than being an actual function.