An expression I don't understand
This is the excerpt of a book Shawshank Redemption.

They are to prison society what the rapist is to the society outside the walls. They're usually long-timers, doing hard bullets for brutal crimes.Their prey is the young, the weak, and the inexperienced... or, as in the case of Andy Dufresne, the weak-looking.

I don't quite understand the very first sentence "They are to prison society what the rapist is to the society outside the walls".
What does this sentence imply?

Question

An expression I don't understand
This is the excerpt of a book Shawshank Redemption.

They are to prison society what the rapist is to the society outside the walls. They're usually long-timers, doing hard bullets for brutal crimes.Their prey is the young, the weak, and the inexperienced... or, as in the case of Andy Dufresne, the weak-looking.

I don't quite understand the very first sentence "They are to prison society what the rapist is to the society outside the walls".
What does this sentence imply?

Answer

He is saying there are men in prison that consistantly rape the other weak or new inmates.

Answer

"They" have the same position within "prison society" that "the rapist" has to "society outside the [prison] walls".
This type of comparison is often used in English. Here's a more simple example:

"A bike is to a biker what a car is to a driver." OR
"A bike is to a biker as a car is to a driver."
This is to say that a bike and a biker have the same relation that a car and a driver have.