Based on a recent quote from Steven Reid, we are given some clarification as to what Early Game Access entails. As most of us maybe used to, when you pre-order a game, typically you'll be able to login and play a few days early.

Bioware is taking a slightly different approach, while yes all of the pre-orders will be able to login early, it's being based upon a first come first serve strategy. This many seem a bit frustrating to some folks, but don't worry. We are given a clarification as to why in the following quote.

While we understand all of you want to be in Early Game Access on Day One, part of the point of a head start program like this one is to allow us to gradually ramp up the population of our servers so that on launch day, the general play experience is as pleasant as possible. While we're going to be allowing a lot of people into Early Game Access at regular intervals, we cannot allow everyone who pre-orders in at once. That would defeat the point of a gradual head start.

The next obvious question is "I pre-ordered on Day X, when do I get access?" While we cannot answer that for everyone, I will say (as I did in July) that those who pre-ordered early will be getting Early Game Access, well, earliest. At this point we still have pre-orders available and therefore, anyone who pre-orders will still gain Early Game Access, but as they're redeeming their code later, they'll get Early Game Access later. As I've said before the difference between 'early' and 'later' should be a matter of days, not weeks

 What does this exactly entail? Glad you asked! By gradually letting in an influx of players, we'll be given less server side issues hopefully and with that a smoother first day experience. I'm sure we can all recount a few mmorpg's in the past where day one, was riddled with bugs. Let's hope that this strategy works and  we'll have a grand experience in a galaxy far, far away!

Source:swtor.com