When a substance dissolves in water it forms a solution. The concentration of the solution tells you how much of the dissolved substance is present in a given volume of the solution. It is often measured in mol/dm3 or in g/dm3.
A concentrated solution has a large amount of the dissolved substance in a given volume of solution. A dilute solution has only a small amount of the dissolved substance in the same volume of solution.
Adding water to a concentrated solution makes it more dilute. The amount of dissolved substance stays the same, but it is now spread through a larger volume. Dilute and concentrated describe how much substance is dissolved. They do not tell you whether an acid is strong or weak.
