As others have indicated, American cities often have their streets laid out on a grid-like pattern. A 'block' is the distance from one street intersection to the next.
However the size of a block isn't fixed. For example, someone posting on this forum states that their city has about 8 blocks per mile (i.e. one 'block' equals about 220 yards). Another contributor suggests that a more usual layout is 12 blocks per mile (i.e. one 'block' equals roughly 150 yards):
http://answers.yahoo.com/question/index?qid=20 071126230823AAzBgMZ
Taking a rough average, it seems that a 'block' is equal to around two minutes walking. (I wish I'd have known that, a few years ago, when an American couple asked me for directions in London. I told them exactly which way they needed to go but I was completely stumped when they asked "So how many blocks is that?").
Chris