Margin trading - When a trader or investor only deposit a part of their total exposure for either buying or selling a stock, forex or commodity. Rest of the money can be provided by the broker in return of a fee, commission or brokerage.
In other words - Buying on margin is borrowing money from a broker to purchase that underlying asset. It is exactly like a loan from a broker to its customers. This can also be referred as leverage trading.
Margin trading might not be suitable for all as it carries high risk of loss while trading. If you are in the right investment then margin trading can maximise your profit substantially but getting caught in the wrong side can also exceed your losses than the initial deposit.