This was an exam question way back when I was a boy doing initial aircraft training.
Decalage is the difference in angle of attack/incidence of the two wings in a biplane.
(Cardinal says the term relates to the difference between main wing and stab - maybe but the term was originally used for bi or triplanes)
If the top wings chord line has a greater angle than the lower one then you have 'positive decalage' If the lower has more AOA/incidence then you have negative decalage.
As D'vay alludes to, an aircraft with positive decalage (i believe negative decalage aircraft are rare) will produce less lift from the lower wing than from the upper. (of course assuming steady level flight and the wings are of the same construction/profile/area/aspect ratio etc etc)
