Willy Harris is of course the name of Walter's "friend" who actually steals his money that they were supposedly going to invest in running a liquor store and runs away with it. This is of course an action that threatens the dream of the family, because it represented the last of the money that the Younger family received from their father's insurance money. Mama has already bought a house and put some aside for Beneatha, but at the same time she has come to the conclusion that she needs to give Walter, her son, the chance to be the man of the family and to take responsibility for things that she, as the matriarchal figure, has previously taken responsibility for. As a result, for Walter to lose the money could potentially destroy the dream of the family because of the way it would create massive conflict and tension amongst them. Note how Walter responds to the discovery of what Willy has done:
Willy! ... Willy ... don't do it... Please don't do it.... Man, not with that money... Man, please, not with that money.... Oh, God... Don't let it be true...
Willy Harris therefore represents a barrier, or something that the family have to overcome in order to pursue and achieve their dream of living together as a family in unity.