Exactly what Trailblazerss said...
LT = Free flowing air compaired to Stock headers, Each exhaust port has its own flowing port chamber all the way down to the collector, which the collectors are usually much wider and better flowing also.
Stock = Exhaust gases leave the port on the head and Instantly get crammed in with the other ports exhausts causing the flow to me much more restricted!
Guess that is the best way I can explain it (Technically)
If anybody plans on making good bolt on power that is a must do!
More air in, More air out!