The short and easy answer is that "roll-crimping" is what your RCBS dies do when the seating dies is set properly.
All it means is that the case mouth is rolled into the crimp groove on the bullet.
The degree of crimp is where the terms "light" or "heavy" crimp come into play, the more the crimp the heavier the crimp.
Does that make sense???
As a general rule, the hotter the load the heavier the crimp, this is to allow a more even and complete powder burn.
In my plinking rounds, 8 to 9 Gr of unique and a lead semi-wadcutter, roughly 800 FPS, I don't use any crimp.
in the heavy hunting load, 24 grains of 296, hardcast 240 semi-wadcutter MAX LOAD, I use a heavy crimp.