It is the amount of weight, per sq.ft. that the wing has to lift. The ideal wingloading goes up as the size of the plane goes up (it has to do with what's called reynold's numbers). For example, a light wingloading for a typical foamy is usually 4-6 ounces/sq.ft, while a light wingloading for a 35% aerobatic ship is around 30-32 ounces/sq.ft. As the wingloading goes up for a given size plane, the plane will have to fly faster and/or at a higher angle of attack, it's stall speed will be higher, it will give you less warning before it snaps out, and will be more likely to snap out in a high-speed stall.
To find the wingloading, divide the wing's total area by 144 to get the total sq.ft and record it. Then take the weight and convert it to ounces by dividing it by 16 if it is not already figured in ounces. Then take that figure and divide it by the sq.ft figure you recorded and you have your wingloading.