H2S(aq) refers to hydrosulfuric acid - a weak acid formed when hydrogen sulfide dissolves in water. The (aq) part refers to the aqueous state of the resultant mixture.
In its pure form, hydrogen sulfide is a gas - a foul-smelling gas, which can be detected in the air at very small concentrations. It is also sometimes referred to as swamp gas, for it is a common product of anaerobic breakdown of organic matter in swamps. When dissolved in water, hydrogen sulfide forms a weak acid, which is clear at first, but gradually turns milky as the gas reacts with the oxygen molecules dissolved in the water.