To clarify, a beta reader is someone who provides feedback on a WIP (work in progress), helping an author refine and improve the story or flow of the message.
Beta readers can be invaluable in helping authors to identify holes, typos, and other issues that can be difficult to spot from the author’s own perspective. They can also provide valuable insight into how readers may react to the content, allowing authors to make changes before publishing.
When looking for beta readers, it is important to find people who are willing to provide honest feedback, and who are, ideally, the target reader for your book. After all, if your book is about how to feel less anxious day-to-day but you ask someone who has never felt a tinge of anxiety to be a beta reader, she likely won't be terribly interested in the content or able to provide the most valuable feedback.