In simple terms , you deposit 1 BTC on the exchange and the exchange lends you say 9 BTC so you can now make trades for 10x your deposit , 10 BTC

However , on regular trading you would need for the BTC to go down or up 100% to lose all , or double your btc , but on margin you would only need the price to vary 10% for that

It basically increases both your possible profit and loss
(The multiplier is called leverage , and each exchange set the maximum , the user can chose how much risk is he willing to take)