A narrow QRS complex refers to a QRS duration of less than 120 milliseconds on an electrocardiogram, indicating rapid ventricular depolarisation through the normal His-Purkinje conduction system. Clinically, it suggests that ventricular activation is occurring via the usual conduction pathways, which helps differentiate supraventricular tachycardias from ventricular arrhythmias. For example, narrow QRS complexes are typically seen in atrial fibrillation with normal conduction, guiding appropriate management strategies.