The Beta Reader is your test audience. He or she is a second editor, so to say. They give you feed back on the story itself much like a content editor, but it’s a bit different. I did a story about picking a beta reader before. Here’s a link to look at it. Finding the Best Beta Reader.
Now a beta reader is a key part of the process, but they likely won’t know it. They are your super fan who shows you the reaction of your ideal reader. Let’s you know if the story is coming across to them as they read it.
I’d go more detail, but I’m afraid of beating a dead horse. I feel very passionately about the power of your beta readers. I’ve written a lot about it. So here are links so all my tips on beta readers and beta work in general.