A dilute acid is one that is not very concentrated. You can make an acid solution more dilute by adding water. Note that you have to be careful with the terms strong and weak; very strong acids can also be very dilute. Strong and weak characterize the acid's ability to dissociate in an aqueous solution, independent of concentration.
concentrated acid is an acid solution with a high molar concentration of hydrogen ions. For example, 16 mole hydrochloric acid has a higher concentration than a 0.5 mole solution of the same compound.