Mesomeric forms are the possible locations of electrons at a single instant, if you get what I mean. Like, an adjacent double and single bond in a molecule. The resonance form, however, is a more accurate picture, where the electrons are represented as being present in both the double and the single bond at once. They resonate between the bonds, bouncing back and forth. This is shown by drawing the bonds as a dashed line. If you''re looking for a couple of diagrams showing resonance forms, you might try looking up the structures of nitrogen oxides such as N2O, NO2, N2O5 and NO. They each display resonance somewhere in their bonding...