Easy..
A simile uses either the words "like" or "as" to compare anything...
My grandma ran to the front of the line, like a cheetah, for some tacos. (Very realistic)
and
Garfield is as fast as my parrot!
It's just comparing things..
Riordan absolutely loves making allusions to ancient mythology. In the Lightning Thief for example, when fighting Medusa, Percy avoids looking at her by using a garden sphere. This is an allusion to the myths, when Perseus avoided looking at Medusa by using his own shield like a mirror. That's a simple example. Let me know if you need more.
An article in an encyclopedia.
The mood changes from this outrageous romantic story because of how much they loved eachother, and how they risked faking their deaths for one another. Then it finally lead the mood into a tragedy due to their actual deaths, and how broken and connected everyone became in the end.