Suppose you are in a field filled with a crowd of children and some of them are holding your hand, some are holding your shirt but you have an urge to reach from point A ( one electrode) to point B (another electrode)? In that case you effective size has increased. How easy would it be to travel from A to B, if you were all alone?
Change in conductance due to solvation is somewhat similar. When an ion wishes to travel under the influence of an electric field between two electrodes, solvent molecules like to travel with the ion, making it "bulkier", hence the ion moves with a retarded velocity.
P.S. Solvation comes from the word solvate which means "A more or less loosely bonded complex formed between a dissolved species and the solvent." It is a general term used in physical chemistry.
Even more formally, http://goldbook.iupac.org/terms/view/S05747
"Any stabilizing interaction of a solute (or solute moiety) and the solvent or a similar interaction of solvent with groups of an insoluble material (i.e. the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as chemically more specific effects such as hydrogen bond formation."