belt wrap is just simply the amount the belt raps around something,in this case the pulleys.
now in theory and i suppose practical terms upto a certain point the amount the belt wraps is greater with a samll pulley[s] and less belt rap is with bigger pulleys.
so belt wrap is the degree of angle the belts turn.
now with a bigger pulley or less belt wrap,this should cause a more free effeciant transmission.were vice versa makes it less effeciant