A drying agent is a chemical used to remove water from an organic  compound that is in solution.  In making or isolating chemical compounds they often become contaminated with water.  If you want to dry a compound that is in solution, a saturated (highly concentrated) solution of sodium chloride can be used to remove the extra water.  Water will move from an area of higher concentration (the compound solution) to an area of lower concentration (the saturated salt solution).  You will get 2 layers: the salt solution layer and the layer of "dried " organic compound solution. The pure compound can then be separated from the salt solution layer.  Drying agents can also be solid like Calclium chloride, Potassium chloride, or Magnesium sulfate.  These are all types of salts (ionic compounds where metals are bonded to nonmetals.)  These drying agents are added to the solution of a compound that needs to be dried.  The agents will easily pick up any extra water from the compound solution and become hydrated. The hydrated salts form clumps which can be filtered out or left behind during decanting resulting in a  "dry" compound.  There are good pictures of this at the link below.