In a chemical equation, the state of each substance is shown by a state symbol written in brackets after its formula. The symbol (s) means solid, (l) means liquid and (g) means gas. The symbol (aq) means aqueous, which is a solution: the substance is dissolved in water.
Take the reaction of magnesium with hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). The magnesium is a solid reactant. The hydrochloric acid is a solution in water, so it is (aq). The magnesium chloride dissolves in the water, so it is (aq) too. The hydrogen is a gas, so it is (g).
Be careful with water. Pure water, H2O, is a liquid, so it is written (l). The symbol (aq) is only for a substance dissolved in water. For example, in CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g), the calcium carbonate is a solid, the water is a liquid and the carbon dioxide is a gas. In 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g), the sodium hydroxide is dissolved in water, so it is (aq).
You need to be able to add the correct state symbols to the equations for the reactions in your course.