It's a form of defense whereas the cornerbacks will play inside man coverage on the opposing wr's, forcing the wideouts to go outside on their routes. The Free Safety and the Strong Safety each play a 1/2 zone covering the middle of the field. The Linebackers will also play man to man defense on any running backs coming out of the backfield, and any tight ends that go out for a pass route. The strength of this defense is that it takes away the short to intermediate routes that a lot of ball control offenses will try to run. There are several weaknesses to it, the two most glaring is that the cornerbacks could be succeptible to deep fade routes, crossing routes, or routes that involve a pick play. The other weakness to it is if you are facing a mobile quarterbakc and the linebackers are specifically assigned to cover running backs, tight ends or 3rd wr's in a formation, it could open up a huge running lane for them if the offensive line can keep the defensive line at bay. for a defense like this to really succeed, you need to have a stron and mobile defensive line that are capable of putting pressure on the quarterback without having to use any blitz packages, and you need what are called "press coverage" cornerbacks that are physical enough to knock the receiver out of his intended route and force them to go into the paths of the two safeties playing the 1/2 zone.
hope this helps
