A few of the rides at the parks have Single Rider lines. They use those lines to fill in any empty spots, so if a car holds 6 people and there's a family of 3 and a family of 2, there is one empty seat, so instead of letting the car go with an empty seat, they will take a person from the Single Rider like to fill it. It's a great way to cut down on wait times if you don't mind riding with strangers.
Some rides use the exit as the SRL entrance and in those cases the CM up front will give you a single rider card indicating that you can enter that way. Other rides have separate SRL lines and you don't typically need a card for them. I've never needed a card for Splash, and I didn't get a card for RSR this last weekend. I do get a card at Indy and Screamin' for example.