A-Level Maths - please help! Watch
I'm presuming you have the relevant course book and have had a good go through it?
It may help if you have a more specific question. For instance, what don't you understand about radians? The basic idea is fairly easy. 1 radian is the angle that's made when you move one radius along the circumference. As the circumference is 2*pi*r which is approximately 6r, then 1 radian is ~ 60 degrees. That's it. It defines angle measurement in terms of length, rather than a fairly arbitrary 360 degrees.