Acids are never obtained or used as pure substances. They are mixtures of water and an acidic compound where the acidic compound is dissolved in water. The concentration of an acidic solution is given as the amount of the acidic compound present in the solution divided by the total volume of the solution.
If distilled water is added to an acid the concentration of the acid is reduced. The water increases the total volume of the acidic solution but the amount of the acidic compound present remains the same.
For example, if x liters of distilled water is added to one liter of an acidic solution with a concentration of m moles/liter the concentration of the acid is reduced to m/(x+1) moles/liter.