In simple terms what is a pansystolic heart murmur?

Murmur: When heart pumps, it undergoes contraction (systole) and relaxation (diastole) for each pumping cycle (cardiac cycle). Murmur is abnormal heart sound heard by stethoscope due to turbulence in flow of blood, which can happen during contraction (systole) or relaxation (diastole). Depeding on underlying problems, you can hear a murmur during early, mid, late or throughout (pan) systole or diastole.

Heart murmur: A heart murmur is sound produced by an abnormal heart structure. Pansystolic murmur is one which continues throughout the whole heart contraction phase. The commonest pansystolic murmur is mitral insufficiency, others are ventricular septal defect and tricuspid insufficiency. There are others less common.