Sine Wave - Introduction to Y = A Sin(wt)
Wtf? Wt is the angular velocity of a sinusoid. It tells us how fast the hypotenuse tip is spinning anticlockwise per second.
We then multiply this 'angle per second' value by time, t, to get an angle spun-through at an instant that we want to know.
Simples :)
Comment with questions and I'll try to help you.