What is a Beta Reader?
A beta reader is someone who reads your manuscript and gives you feedback on it. They read it like they would read a recreational book. They aren't editors who are reading each sentence word for word. They simply read it. They report back afterwards and tell you all the kinks that need to be worked out. Any inconsistencies, editing errors or plot problems, they tell you. They can give you feedback on the flow or on certain characters.
Why do I need one?
I know from experience that reading your own material, you ten to miss things. After draft three of my book, I am still finding errors that I missed before. A beta reader can only help. They will find things that we miss time and time again. Sometimes we get so excited for the main conflict that we rush through it. Maybe because we have this idea of a character, we imagine them in our head but don't write it down fully.