So from what I can gather, a beta reader is someone who reads your work before it goes to the publisher. It can be another author, or a trusted friend. They give honest and open feedback that will often change the direction of the story, before it goes to the publisher.
While writing The Exile, I shared the first part with my wife only. I was pretty worried about and insecure about my writing and I knew she would be gentle with her feedback. After getting feedback from her and making some huge changes to my book, I then started reaching out to other people for feedback.
I know that grammar is not my strong suit so I first shared my work with my father-in-law. The feedback I received from him has been irreplaceable. He pointed out inconsistencies, asked good questions, and suggested some rewording for clarity. He actually didn't do too much with the grammar or punctuation of the book, but instead focused on the content.
Before I sent the book to publishers I also had a handful of other people view parts of the book and a very select few saw the entire book. I asked for feedback about the content, characters, setting, and overall story line. Some of the feedback I received I did not end up using, but many of the things people said, helped a lot.
When I first started sharing my writing, I was scared by what people would think about it. Honestly, some times I still feel that way now that the book is available. I was also somewhat hesitant to get feedback from people because I liked my story the way it was. By sharing it with others I came to realize that sharing my work helped to overcome both of these feelings. I am now getting more and more excited to share my book with people, and I am happy when I get constructive feedback that helps my stories become better.
I honestly don't think my book would have made it very far without the wonderful people who read and gave feedback on it when it was in the first stages.