Limit order is a way to buy or sell a certain coin/token at the desired price. Funds required for the execution of this order (total cost of the transaction) will be blocked in your account. They will be debited from your account as soon as the market price reaches the one you set and the order is executed.
In order to do so, you have to indicate the price at which you want to buy or sell, the desired amount of cryptocurrency, and click the Buy or Sell button.