They refer to the average excess (or otherwise) of fuel carried on that sector over the previous 3 months. It gives an indication of any likely additional fuel requirements due to e.g. holding, routings etc.. Extra fuel shown here is a good indicator that you too will need some extra fuel for the leg, as your contingency fuel is for 'unforseen variations' (FCO1750) to the operation. Clearly if an excess is indicated on SWORD, then the variation is not unforseen.
Hope that helps