That means what happens in previous races will carry through to the next, which sounds like something of a linear progression (as opposed to, say, Pacific Rift's festival, in which you ran a bunch of races to unlock the remainder in the series, and went back and raced them for higher medals if necessary.) Evolution Studios says the races will be more accessible at the beginning, then very difficult in the end, which sounds about as I remember MotorStorm: Pacific Rift.
MotorStorm: Apocalypse also will feature new vehicle classes (now 13, up from 8). One final fun fact: The composer behind the Pirates of the Caribbean soundtrack was brought in to do MotorStorm: Apocalypse's. The game is due "early 2011." Enjoy.
MotorStorm Apocalypse Dev Diary Details The Festival, New Gameplay Video [PlayStation Blog]