This is the title of a chapter in a book for young people. What does it mean? The sentence is: "She's a missionary to the rich."
This chapter is about a young, christian girl who has moved to USA from Mexico. She is poor. She receives at one occasion a lot of food that she doesn't want to keep for herself. So she gives away some of the food to her poor neighbors. She also tells them that they are loved by God. I think there is another, hidden, meaning in this title.
Here is a definition of missionary.
It means someone who is on a mission, that is they try to convince people of something, or to convert them to a religion.
"to the rich" means that wealthy people are the object of the efforts of convincing or converting.
Since this is the title of a book chapter, it may be necessary to read that chapter to fully understand what is meant.
A missionary is:
a person who has been sent to a foreign country to teach their religion to the people who live there
(The) rich as a noun means:
rich people considered together as a group
So a "missionary to the rich" is a metaphor for a person who is in some way attempting to teach some kind of principles to rich people. Since you didn't specify the name of the book it's impossible to derive any more specific meaning from the sentence, but hopefully this now makes more sense to you in context.
