...and meaning. How is it triggered and how is it important?

:)

Basically how clean a rec. can get off the line. (in a nut shell)

This is just the counter to a DB's B&R coverage. When a DB plays B&R he will jam the receiver at the LOS, how effective he is at slowing the receiver down by doing that is driven by this difference. If the B&R avoidance is higher than the B&R coverage attribute, then the DB will have little to no effect on the WR's route most of the time.

In case you don't know, B&R is Bump and Run coverage. (definition)