A margin call is triggered when your account equity falls below your margin requirement (thereby creating a negative available margin balance). When notified of a margin call, you will have a stated deadline to respond and rectify the deficit. This can be accomplished by either adding cash or selling securities. In extreme cases, assets may be liquidated prior to the deadline.
A decrease in the value of your stock positions, or withdrawals of cash using margin will move your account equity closer to your margin requirement and therefore increase your likelihood of receiving a margin call. Similarly, an increase in the margin maintenance requirement of a stock you already hold can lead to a margin call in your account.