Emerald Aisle is National's program that allows you to skip the rental counter. It's a great timesaver. When you sign up, you will put in all your info and preferences (whether you want the insurance or not, lic #, etc) and they will be stored. You make your reservation online through National's website...just put in your EA #. Then when you get to Orlando, just go to the garage and there will be a clearly marked section of cars for EA members. Just get in whatever one you want, drive to the booth, show your EA card, license and any coupons you've used, and drive off.
When you return the car, you pull into the garage, they come check the mileage & gas tank (make sure you fill it on the way back) and hand you a receipt and you're off to your flight.
Keep in mind that most of the cars on the EA are Intermediate or Full size and if you have a special requirement (like a minivan), don't go there counting on getting one. What's available all depends on what time you get there, day of week, etc.
Definitely worth it...especially because it's free!