Emerald Aisle is a service that is available to members of the Emerald Club. You would need to join the Emerald Club to use that feature. National used to have free membership available to the EC via their website, but that is no longer available. From what I understand you can get a free membership to the Emerald Club through the Disney Club. You would probably need to go through the Disney Club website to get the how to's on this, or maybe someone here can tell you more about how to join the Emerald Club with your DC membership.
When you reserve a fullsize or midsize car through National using your Emerald club membership, you will, at most locations, have the option to chose Emerald Aisle service. With Emerald Aisle service you will be able to bypass the counter at the airport and go directly to where the cars are located. You can chose any car in the Emerald Aisle. Sometimes they have SUV's, convertables, MiniVans and other specialty vehicles on the Emerald Aisle. Other times they do not. It is kind of a luck of the draw situation. You will always be guaranteed at least what you reserved.
Once you pick a car on the EA you can load it up and drive it to the checkout gate where they will ask you for ID and reservation information and check you out there. It is a big timesaver. There are other features of Emerald Club as well, but I don't know how much detail you want or need. I hope this helps.