A beta reader is usually an unpaid test reader of an unreleased work of literature or other writing, who gives feedback from the point of view of an average reader to the author. A beta reader is not a professional and can therefore provide advice and comments in the opinions of an average reader. This feedback is used by the writer to fix remaining issues with plot, pacing, and consistency. The beta reader also serves as a sounding board to see if the book has had the intended emotional impact.
extracts from Wikipedia