What single riders means, is that you are willing to ride by yourself. As staff try to fill the carts with people, not everyone has a group of the right size and they will use single riders to fill in the remaining seats.
As long as you are willing to ride by yourself, this is a great way to avoid the wait times of long lines. But in the case of some rides you may miss some of the cool things they have in the wait lines, such as Hogwarts Castle. But don’t fear, a lot of places also allow people a walk through who are not intending to ride the ride at all. This way you can always come back and see some of the stuff in the wait line, without have to waste your time waiting for the ride.