Beta readers are the last eyes on your book. They tell you what works, what doesn't, and give suggestions on how to improve. Much like an editor. The difference is, typically, beta readers are free.
Make sure you find beta readers who read your genre. You don't want people expecting a happily ever after when you're writing a thriller.
Scribophile is a great place to find beta readers. You can join various groups, post your writing to get feedback (after earning enough karma points by critiquing others' works) and join novel swaps.
At the end of the day, you want people who understand your vision for the story and don't try to change your voice. After all, it's your story.
Lucia Sugafield, enjoying the process