A radian is 2π, a circumference is 2πR, and the angle of a circle is 360, corresponding to 1rad=( 180/π). If the radian of a sector is a, then the arc length of this sector is A*R/2π, and radian plays an important role in trigonometric function. It doesn't matter if you don't understand it now, you will understand it when you actually use it.