This is the amount of money Henchard received from the sailor in exchange for his wife, Susan. A guinea is a unit of British currency (now no longer used) equal to one British pound plus one shilling. It was a lot of money back in the late 19th century. So five guineas was a whole lot of money for a poor man back in the day.
Michael Henchard is very into symbols. We're told in the second chapter that there is "something fetichistic in this man's beliefs" (2.7). So he actually turns the five guineas into a symbol himself by making a big show of giving Susan five guineas when they're reunited. He is symbolically "buying her back." ("The amount was significant; it may tacitly have told her that he bought her back again" [10.43].)