The spread offense has become more and more common in football as more and more teams have apparently decided passing the ball is sexier than running it. Regardless of why, the spread literally means spread out. Since the team can’t spread ‘vertically’, spreading out really just means having lots of receivers as wide as the field allows. It usually results in passing plays, but again, not always. They will often keep one back with the QB to block when he is passing (after taking a fake handoff to keep the defense thinking) and take the ball for a run play every now and again too.
In the diagram above, notice how many receivers and how wide they are on the field. The spread offense usually has 4-5 receivers all over the line. If the offense also has a running back and/or a good running quarterback as well as some really sure-handed receivers, this is hard to defend.