Early Action is just like regular admissions...only you do it earlier. You have an earlier deadline, and you get an answer back early. Whereas most regular application deadlines are somewhere around January 1, most Early Action deadlines are around October 1. And instead of getting a decision back in March, you get it in December. It's not Early Decision--you aren't committed to going to the school if you get accepted. You have until May 1 to make your decision, and you're welcome to turn them down even though they accepted you Early Action.
Not all universities offer an Early Decision option, and each one might have its own fun little stipulations and rules. But the basic idea of Early Decision is that you turn in your application early, you get a decision from the school early, and if you get accepted you agree to go there and withdraw any other applications you may have also sent. This requirement that you enroll if you're accepted is why Early Decision is usually referred to as "binding." Early Decision is often confused with Early Action, which I'll write about separately next week. But for today, remember that Early Decision is exactly what it sounds like: you decide early that you really want to go this school; they decide early if they're going to accept you; if they do, then it's decided--early--that you will definitely go there.