Radians are a unit of angle measurement that is mathematically more convenient than degrees. Where angles specified in degrees range from 0 to 360, angles specified in radians range from 0 to 2 Pi, with 0 radians measured along the positive X axis and increasing counter clockwise.
This puts the positive Y axis (90 degrees) at Pi/2 radians, the negative X axis (180 degrees) at Pi radians, and the negative Y axis (270 degrees) at 3 Pi/2 radians. If you (or your application) are more comfortable with degrees, radians can be converted to degrees by multiplying a radian value by 57.2958. To convert degrees to radians, multiply by pi/180.