Molarity is one of the measures of the molar concentration of a solution. It is calculated by dividing the number of moles of the solute by the volume of the solvent. This means that the higher the amount of solute in relation to the volume of solvent, the higher the molarity of that solution.
In essence, adding water to a solution dilutes it i.e it increases the solvent's volume in relation to the solute, causing the molarity to decrease. In a nutshell, diluting a solution (by adding water or more solvent) causes the molarity of such solution to decrease. For example, if water is added to a 0.70 molar solution of CuSO4, the molarity of the solution will DECREASE.
