Star Wars: The Old Republic early access explained

The ‘Early Game Access’ offered to those who’ve ordered Star Wars: The Old Republic prior to release has caused some minor confusion, so BioWare has moved to clarify matters.
First of all, nobody knows precisely when ‘Early Game Access’ (EGA) will begin. If BioWare themselves know, they’re not yet saying. It’ll be before the release dates of 20 December (US) and 22 December (Europe) though.
Everyone who already ordered Star Wars: The Old Republic will be eligible for EGA, but entrance will be “determined in a first-come, first-served manner dependent on when you redeemed your pre-order code.”
For example, there are still copies of the game available to order. Doing so will make you eligible for EGA, but you’ll be behind the people who already ordered and redeemed their code.
However, BioWare do say that “we’re going to be allowing a lot of people into Early Game Access at regular intervals” and that “the difference between ‘early’ and ‘later’ should be a matter of days, not weeks”.
Access will be staggered to gradually increase pressure on the game servers, without overloading them.
Source: swtor.com