Lucien is an alien, from an alliance of different alien civilizations called the Federation, all of which are capable of faster-than-light travel using something called warp drive and most of which are spread across multiple planets. His first time at warp on such a ship on his way to Starfleet academy went very wrong until it caused him to meet Annie, which definitely feels worth it now because - maybe he should save that for later, there are other very important secrets he can share right now instead.
The Federation has a strict policy of not contacting civilizations like this one that haven't discovered warp drive, which is a huge humanitarian tragedy that he wanted to change from within or at least push a bit towards a better direction that didn't let so many people die of preventable causes. The reason she might want to keep it secret is that if they are near enough to the Federation to be discovered, then it would be good for the Federation to be under the impression that this civilization discovered warp on its own and not with his help.
He can go into the basics of warp drives and subspace next, but he'll need writing implements to actually explain the details he can remember in a way that thoroughly communicates them - also has her planet discovered that space and time are sort of intricately related and very massive objects can bend both and such? That will help with understanding it.
Also, his species is human, and the planet of origin is Earth. That's uh, the first few secrets, does she have questions or anything?