Leverage and margin is a very important concept to understand, because leverage can get you in trouble pretty quickly if not used properly. That being said, if leverage is applied properly, it can increase the profitability of your trading strategies.
Leverage trading, or trading on margin, means you aren't required to put up the full value of the position. As a result, you can open a significantly larger position that you would be able to if you needed to fund your trade in full. Trading on leverage increases your potential for profit, but also increases your risks.
Forex trading offers leverage up to 200:1, This means that for every £1 in your account, you can trade £200 worth of a position.