A radian is a unit of an angle at the center of the circle. The arc's length is equal length to the radius. Radians relate to the unit circle because they can be converted into degree measures around a circle. These measurements help us find the reference angle, the sines, the cosines, and the tangents of a triangle. The circumference of a unit circle is 2pi radians or 360 degrees. With this info we can figure out how many times the radius can go around the circle with the respected amount of radians given. Radians and degrees can be converted to each other. I prefer degrees because it gives you a nice whole number that I can visually see better. Radians tend to be fractions with pi attached to it, making the measurement look gross. However, radians are more mathematically pure.