You can read more in his posting Long Tail 101 or in the wikipedia entry.
The theory of the Long Tail is that our culture and economy is increasingly shifting away from a focus on a relatively small number of "hits" (mainstream products and markets) at the head of the demand curve and toward a huge number of niches in the tail. As the costs of production and distribution fall, especially online, there is now less need to lump products and consumers into one-size-fits-all containers. In an era without the constraints of physical shelf space and other bottlenecks of distribution, narrowly-target goods and services can be as economically attractive as mainstream fare.
Most of us normal blogger folks here in the wacky wide wonderful world of the blogosphere are "long tailers".
Long live the long tailers!
Read more of Chris Anderson here!