Angles are often thought of in terms of circles. Every angle can be described as a fraction of the circumference of a circle. This, combined with the fact that the circumference of a circle is equal to 2πr, allows for every angle to be described by the radius of the circle. Measuring an angle in terms of the radius of a circle is reffered to by the unit radians.

This video made by Lucas V. Barbosa is a great demonstration of what a radian is.