roll-off means isolating the 100hz frequency and using the low-shelf eq which
attenuates all frequencies below that number as you decrease the gain in db. Alternately, you could use a hi-pass eq, which will essentially cut these same frequencies in a more drastic fashion, thus completely eliminating them. That's referred to as a "lo-cut".
If you have any eq plugs that use a visual graph, like the ones from Waves (as opposed to a purely numerical one on the Digirack), you could actually see how this works and understand it better. Hope this helps somewhat.
For more on this and anything to do with recording, here's the holy grail of sites.
www.studiocovers.com
Have fun learning.