I've been asked the question "what is a zone defense?" so allow me to enlighten you...
In simplest terms, a zone defense is using your players to cover areas of the fields (or "zones"), rather than covering each person one on one.
The primary advantage is that it allows you to take what might be a bunch of average players and have them help each other cover the field, rather than asking them to cover a player who might be above average.
In the example above, we might have 6 players in a zone that covers the field, and that leaves 5 players to rush the quarterback. OR, one of those players might drop into coverage and either help in a zone, or might pick up a particular player man to man to make the zone more effective.
The challenge is that in covering areas of the field, you might have a seam in the zone. Think of a bunch of pieces of fabric covering the field. Where that fabric comes together is a seam. And if the offense manages to get a player to the back part of one area, and another player to the front part of an adjoining area, there is a seam because no one is covering that part.
Need a little more detail? The idea is you utilize one player in the zone. But, lets say that both of the receivers at the top of the picture run short curl ins, so they are in zone 3. Maybe the running back goes out for a pass into zone 4. So, now you've got two people in zone 3, and 1 in zone 4. The guy in 3 has to cover two people, and the guy in 4 can't help him, because he's occupied. The guy in 6 could come up, but now there's no help deep should someone suddenly turn upfield to that side.
So, the offense might gain an advantage. Or they might send a receiver to zones 3&4, and then send the back into 4, while edging the players deeper toward 6. Again, the idea for the offense is to try and get the advantage against this.
And frequently, an agressive offense can get some advantage and complete short passes into the zones, and keep the defense on the field, much as we saw on Sunday. But as the field gets shorter and shorter, there is less area to defend, and it gets easier for the defense...
Of course, the defense can add a level of sophistication to this. There are 6 players out there. One of them may be switched to man-man coverage, and the other 5 will adjust to cover more of the field. Or maybe one of them comes up to blitz the QB, and the others control the field.
Its all about trying to make the most of your personnel...