What is a margin call?
If your broker's margin requirement stands at 5%, this share of the transaction size must always be maintained. So, if the value of your position decreases and your 5% effectively becomes worth 4%, you will be required to fund the additional percentage point in order to meet this minimum requirement. If you fall below this threshold level, you will be required to stump up the extra funds and you may even experience the margin call - a demand from the broker for funds that has the effect of closing other open positions to liquidate your portfolio. This can naturally strangle off otherwise successful positions, and can end up costing you heavily, so it's best avoided at all costs by keeping sufficient reserve capital in your account.