When you’ve done as much as you can on your manuscript and you think you’ve got it down to the “final” draft, you’ll need the opinions of people who haven’t seen all the drafts that came before to tell you in an unbiased way what’s good, and what could be better. Enter the beta readers.
Beta readers don’t have to be writers, they just have to be readers. Preferably readers who enjoy the genre you’re currently writing in. They read your story, as is, in full, and give you feedback. They have a better chance of noticing any problem areas than you at that point because they haven’t been obsessively developing and changing the book for who knows how long.
They have the distance from the project that you never will to notice when a plot was never resolved, a character wasn’t developed, or a character randomly appears or disappears out of existence with no acknowledgment. The big picture, the details, how your theme came across, if they liked your characters, if anything was confusing or over-explained. Everything.
Beta readers can be anyone, but if you’re asking friends and family, make sure they’re being completely honest and not glossing over the bad stuff just because they love you and they’re so proud of you and what you’ve accomplished. Also, don’t take it personally if someone you love doesn’t love everything about the thing you made. While you don’t have to change everything your beta readers critique, rejecting feedback entirely isn’t going to help you improve.