When an acid dissolves in water, its molecules can dissociate, which means they split up to release hydrogen ions, H+. The strength of an acid depends on the degree of dissociation into ions.
A strong acid is completely dissociated into ions in solution. Every molecule releases an H+ ion. Hydrochloric acid, nitric acid and sulfuric acid are strong acids.
A weak acid is only partially dissociated into ions in solution. Only some of the molecules release H+ ions and most stay as whole molecules. Ethanoic acid and citric acid are weak acids.
If a strong acid and a weak acid have the same concentration, the strong acid releases more hydrogen ions, so it has the lower pH. Do not confuse strength with concentration. Concentration is about how much acid is dissolved. Strength is about how much of it dissociates.
