prime lap (prounouced PREEM) is a race within a race - whoever wins that particular prime can quit the race and collect the prime. Sometimes it's a field prime, if there's a group ahead. Other times it's a "first over the line". It can even be for solo riders, like at a big race sometimes a hardy fighter, off the back, may get a prime thrown at him as a mercy prime ("20 bucks if he can last another lap!").
*edit* since it's not the actual finish, fewer people contest primes. Therefore it's a good way to break into "contesting stuff". You go for a prime, figure out how win them, then you go for the win.