It lets you create Romulan characters in all-new story-driven missions. This also adds Warbird Class starships to captain. There are also new improved traits and an improved UI.
The latest STO blog post also looks at the new planet Nimbus which is a haven for criminal elements. It's a level 21 to 25 adventure zone available to all factions.
To find out exactly what will be invading your Holodeck next week, check out the official Star Trek Online website to learn more about the Legacy of Romulus expansion.