Resonance structures show molecules like benzene and some amino acids (histidine, with its imidazole side chain, for instance) in their various forms that they are found naturally. These molecules are able to adopt different electron distributions without changing the position of atoms in space.
Occasionally, a curved line (or circle in the case of benzene) is drawn to show there are multiple positions that electrons could adopt in a molecule. This includes deprotonated carboxylic acids (–COO-)