The singles rider line means that your party will be spilt up into parties of one. You will fill in empty single seats as they become available. You can save a lot of time by using the single riders line. Once in June, I rode the Hulk seven times in 45 minutes. The regular wait was 45 minutes.
The following rides have single rider lines:
IOA: Spider-Man, Hulk, Fearfall
USF: MiB, E.T.
Single rider lines open and close, depending on the queue. MiB generally has a single rider line after Universal Express is done.
The moderator formerly known as Barry aka usagi
Moderator, Universal and Sea World Boards