In the electrolysis of aqueous NaCl, hydrogen gas (H$_2$) is liberated at the cathode, and chlorine gas (Cl$_2$) is liberated at the anode. This is because chlorine has a higher oxidation potential than water, which leads to the oxidation of chloride ions to chlorine gas at the anode. Hence, both the assertion and reason are true, and the reason correctly explains the assertion.