Simple to say, Margin actually means bank borrow money for you to buy shares, but bank need protect them-self also because they cant afford let the client keep holding the shares if price drop, so they have a calculation to determine whether client get margin call or not, the calculation is amount borrowed / current share price, so if the share price drop, the margin value will getting higher until you reach their threshold like for cimb is 70%, then this is margin call and remiser will inform you to sell or deposit more money