Limit buy orders are only executed when the buy price (ask) reaches, undershoots or already lies under the limit price.
The price of a stock is currently at 100 euros. The trader, however, only wants to pay 98 euros for this stock. He/she thus enters a limit buy order with an execution price of 98 euros for that stock. The order will not be executed until the stock falls to 98 euros or below. As long as the stock is listed above 98 euros, the order remains in the system until the end of the validity.