A beta-reader is a person who reads a work of fiction with a critical eye, with the aim of improving grammar, spelling, characterization, and general style of a story prior to its release to the general public. (definition from Wikipedia)
Members who need a beta-reader can contact one if they need help. To see the list of beta-readers, go to "Authors" (see navigation on the top) and select "Beta-Reader". To contact a beta-reader, go to his profile and click on "Contact Member".