In general terms, Defense represents the quality issues of a company. Offense represents the value issues of a company.

To expand on that, slightly:

Defense is talking about companies that are not meeting the expectations set by performing a stock study, or by comparing current resutls to historical averages.

Offense is talking about companies that appear to be overvalued for one reason or another and, perhaps, should be replaced with companies that can provide a more desirable total return.