A solution forms when a solute dissolves in a solvent, which is usually water. The words dilute and concentrated describe how much solute is dissolved in a given volume of solution.
A concentrated solution has a large amount of solute in each unit of volume. A dilute solution has a small amount of solute in the same volume. Concentration is measured in grams per decimetre cubed (g/dm3) or in moles per decimetre cubed (mol/dm3). A solution containing 2 mol/dm3 of an acid is more concentrated than one containing 0.1 mol/dm3 of the same acid.
Adding water makes a solution more dilute, because the same amount of solute is now spread through a larger volume. If a solution is mixed with an equal volume of water, the volume doubles and the concentration halves.
Dilute and concentrated describe only the amount of solute in the solution. They do not tell you whether an acid is strong or weak.