A limit order is an order on which you manually indicate the quantity and price at which you wish to buy or to sell. The aim is to sell higher or buy lower than the current market price. Thus, the order is not executed instantaneously, you have to wait until the price reaches the order price before it is executed. This can take minutes, months or even never happen. For example, if I place a limit order to buy a Bitcoin at €10 now, chances are it will never happen.
Please note that all orders are visible in the order book.