A reference angle is the acute angle (<90) formed with the x-axis and the larger angle.
For example, a 220 degree angle can be represented by drawing a ray on the positive x-axis and a ray in the third quadrant. The ray in the third quadrant makes an acute angle of 40 with the x-axis. If you draw this out you should understand it pretty easily. The reference angle is always less than 90 degrees and it is between the x-axis and the second ray(the one not on the x-axis) in the larger angle.
Here are some of the formulas you need - your book should list the subtraction ones (or just let v be negative) - or search on wikipedia
sin(u+v) = cos(u)sin(v) + sin(u)cos(v)
cos(u+v) = cos(u)cos(v) - sin(u)sin(v)
Here is an example:
let u=80, let v=10