Kronos is the Titan Lord. The Titans are gods that existed before the current Greek gods. Kronos is super, super powerful. He is the Titan of Time, so he can control time to pause, play, fast forward or rewind it. In other words, he is very powerful.
Percy is the son of Poseidon, the ocean god. He can control water, like making waves and stuff. He can make bubbles in water, and he can go in water without getting wet.
Around the beginning of this story, Percy bathed in the River Styx, the underground river that goes through the Underworld, Hades' domain or kingdom. This made him invincible.
Kronos attacks the world so he can destroy Olympus, the city of the gods where Zeus lives above the Empire State building in New York City. He wants to take over the world.
In the end, it isn't Percy who saves the world. It's Luke, Kronos' spy who used to go to Camp Halfblood with Percy. After he finished spying, he bathed in the River Styx like Percy and Kronos took over his body. Like Achilles, Luke had a weak spot where he stabbed himself so that Kronos would be destroyed. Luke was going to die anyway, so he killed himself to save the world from Kronos.
I think the main idea of the book is that even if you've done things the same way for thousands of years, you can change for the better. Also, you should always accept others even if they're a bit of an oddball or different. I think this because at the very end, when Kronos had been defeated, the gods were granting the demigods who helped defeat them wishes. Percy's wish was that all the demigods be accepted and claimed by their god parents.
I recommend this book to anybody because it's really good. I think it is the best one in the Lightening Thief series. I liked this book because there's tons of action in it, it's got a bunch of cool twists, there's a cool dragon and also a flying pig.
If you want to learn more about Percy and the other books in the Lightening Thief series, read my other reviews:
The Last Olympian (Percy Jackson & the Olympians, Book 5): Rick Riordan: Books