On Oasis and Allure you have to reserve some show times in order to be guaranteed a seat. Those with reservations are seated first, and then when it gets to be 10 minutes from show time they open up the venue for anyone to enter and those who had reservations but didn't arrive in time are on their own too. There is no fee involved. It is a crowd control method because of the passenger loading versus venue seating capacity.
For these shows they offer them more than once during a cruise so all passengers will get to attend the shows, it just may not be at their preferred time or day.
I should add that you can only reserve one seating per show, per cruise. If you want to see it again you have to wait until they open it up to standby guests.