Lets you control a position larger than your capital by depositing only a fraction as margin.
With 1:100 leverage you can open a 100,000 USD position by putting up 1,000 USD of margin. It amplifies both gains and losses.
Leverage does not change the value of a pip or your risk per trade if the lot and stop loss stay the same; it changes how much margin is locked and how easily you can over-expose yourself. High leverage with large lots is one of the most common causes of blown accounts.