Septum is part of the muscle of the left ventricle (LV: the main pumping chamber of the heart). Hypertrophy means thickening of the muscle. Unlike concentric hypertrophy where the whole LV is thickened commonly due to high blood pressure, in Asymmetrical Septal Hypertrophy (ASH, which is genetic) only the septum is thickened which may obstruct the blood being pumped out of the heart. This may be dangerous especially during exercise. But if the echo report mentions only ASH and not obstructive hypertrophic cardiomyopathy this is unlikely to be dangerous (especially if there is no family history of sudden death). There are many minor forms and sometimes a normal variant called Sigmoid septum is reported as ASH. Your doctor can explain your particular case well.