what do you mean by negative and positive enthalpy of hydration?

Enthalpy of hydration is always negative. It is the enthalpy change when one mole of gaseous ions dissolve in sufficient amount of water to give an infinitely dilute solution. The energy is released due to the formation of new bond between ions and water molecules. There is attraction between water molecules and ions (positive or negative ions). Higher the hydration enthalpy means large negative value of  hydration enthalpy. Smaller is the size of ion more heavily it is hydrated and higher its hydration enthalpy. There is no positive enthalpy of hydration.