State symbols tell us the physical state of each substance in an equation. They are written in brackets straight after the formula. (s) means solid, (l) means liquid, (g) means gas and (aq) means aqueous, which is dissolved in water. The symbol (aq) comes from the word aqueous.
Be careful with (l) and (aq). Pure water is H2O(l). Sodium chloride dissolved in water is NaCl(aq), because the sodium chloride is dissolved, not a pure liquid. Steam is H2O(g). A precipitate that forms in a solution is an insoluble solid, so it is shown as (s).
State symbols help us picture what is seen. In Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g), the solid magnesium disappears into the acid and bubbles of hydrogen gas are given off. In CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g), the carbonate reacts with acid and carbon dioxide gas fizzes out.
State symbols are only extra information. They are never used to balance an equation, so they do not change the big numbers in front of formulae.