hydrogen chloride dissolves in water to produce hydrochloric acid. Hydrochloric acid is a strong acid. What is meant by the term strong acid?
A strong acid is one which fully dissociates into its ions when dissolved in water. For HCl, those ions would be H+ and Cl-. Essentially when it's in water all of it becomes hydrogen ions and Chloride ions rather than some of it remaining as HCl and forming an equilibrium like a weak acid would.