Mostly it's about the recolonization of Earth (some hundreds of years after a nuclear apocalypse), via a cross-breeding program devised by the Oankali. Lilith is awakened (having been rescued from the dying planet) and the program is "sold" to her. She then leads the first inhabitants reintroduced to a healed Earth.
The big question is: Is humanity being saved or exploited?
The second novel introduces a plan to colonize Mars with pure humans. The alien Oankali race is betting they'll destroy themselves within a couple generations.
What's compelling about this series is how well the alien species and its culture is imagined. The aliens are completely alien, but they are flesh and blood; their ways are weird, but so natural. And over time, they gradually become more human.
Of course one of the biggest attractions of the book is all the alien sex. Which sounds kind of awesome. The Oankali is a species with three sexes: male, female, and ooloi. They mate together "through" the ooloi; the ooloi establishes a neural link with all parties, and mixes all the genetic material. Now throw in a couple humans, a male and a female, to get the cross-breeding — a genetic trade — underway. Once mated, the parties develop a revulsion for mating with anyone else, to the point of not even being able to comfortably touch one another. They connect only through the ooloi. That neural link must be out of this world.
"This isn't just a drug."
"Direct stimulation of the brain and nervous system." She held up her hand to stop him from speaking. "There's no pain. They hate pain more than we do, because they're more sensitive to it. If they hurt us, they hurt themselves. And there are no armful side effects. Just the opposite. They automatically fix any problems they fin. They get real pleasure from healing or regenerating, and they share that pleasure with us. They weren't as good at repairs before they found us. Regeneration was limited to wound healing. Now they can grow you a new leg if you lose one. They can even regenerate brain and nervous tissue. They learned that from us, believe it or not. We had the ability, and they knew how to use it. They learned by studying our cancers, of all things. It was cancer that made Humanity such a valuable trade partner."
I'm including here a picture of a blobfish because it looks just like I imagine one of the cross-breed subadult ooloi whose metamorphosis goes horribly wrong for a little while when it doesn't have recourse to the grounding influence of mates (but it ends up OK).