In Chapter 10 of Percy Jackson's The Lightning Thief, what is the Mist?
The Mist is a supernatural force that obscures, or hides, other supernatural things like gods and goddesses from the vision of mere mortals. The Mist is a sophisticated tool that can be interpreted in a literal way, as a sort of cloud that blocks human vision, or in a figurative way, as a filter through which to perceive and later, to understand, the reality of challenging events or situations.
In The Lightning Thief, the Mist can be understood both literally and figuratively. Let's take a look at Sally Jackson, Percy's heroic mother, one of the few mortals who can see through the Mist, to better understand these two ways of interpreting the Mist.
Literally, Sally can see through the Mist and observe gods like Poseidon going about their business. This ability is what led her to Poseidon in the first place, with whom she had a relationship and a son, Percy. Figuratively, Sally's ability to see through the Mist correlates directly to her characterization as a reliable mother figure to Percy; her ability to see and understand the world in which they live makes her the best mother that he can imagine.
When Chiron gives Percy the gift from his father, Poseidon, a magical sword named Riptide, Percy worries that mortals will see him wielding a giant sword and freak out. However, Chiron assures him that Mist is very powerful, and this Mist will generally prevent mortals from seeing the truth about things and events having to do with gods or divine objects. Chiron says that it is "'Remarkable, really, the lengths to which humans will go to fit things into their version of reality.'" What he seems to mean by this is that human beings need the world to work the way we already conceive it to, and since we don't believe that someone can uncap a pen and suddenly be holding a huge sword, we simply won't be able to see that this is the reality. Therefore, our minds simply replace things that we don't think could be real with things that we believe can be; for example, instead of seeing Percy holding Riptide, we might see him holding a baseball bat because this fits with our ideas about reality.