From the EQ2 Forums:
What is a Time Locked Expansion server?
A TLE server has a special ruleset which restricts content and allows players to revisit exciting adventures from previous eras of EverQuest II. Every 12 weeks, content from the next expansion will unlock allowing progression through much of the original game content over time.
Who can play on a TLE server?
All Access Membership is required to create characters and play on the Fallen Gate server.
How will content unlocking work?
Every approximately 12 weeks, a new expansion and it associated content will unlock on the server. Adventure packs such as the Fallen Dynasty will release 6 weeks into the applicable expansion.