Okay kiddo, so you know when you go to the store with Mom or Dad and they have a list of things they need to buy? They could go to any store to buy those things, but sometimes they like to go to a specific store because it's easier or cheaper or they have a special deal there.
Well, it's kind of the same thing with a preferred pharmacy network. When you need to get medicine, your mom or dad could go to any pharmacy to get it, but sometimes their health insurance plan has a list of preferred pharmacies that they should go to. These pharmacies might be easier to get to or they might be cheaper because they have a special deal with the insurance plan.
Think of it like having a special list of stores that Mom and Dad should go to for certain things. That's what a preferred pharmacy network is!