Chromatography is a separation method that involves the use of a stationary phase and a mobile phase. The stationary phase is immobile, in the particular instance of this question, the stationary phase is paper. The mobile phase is the appropriate solvent, in this case, a salt-water solution.
If the level of solvent is above the dye spots, it will introduce error into the separation. The solvent (if volatile) may evaporate without drawing up and separating the solute. Secondly, the solvent may simply dissolve the spots without achieving any meaningful separation of the components in the system. This second reason is particularly why the salt solution must be below the dye spots in this chromatographic separation.
