A Limit Order is a type of order to buy or sell an Asset at a specific price. For purchase limit orders, the order will be executed only at the limit or lower price, while, for sales limit orders, the order will be executed only at the limit or higher price.
Example: If you want to exchange 2 Bitcoin (BTC) for Tether (USDT) through a Limit Order, the exchange rate is set (in this example) with a limit of 9000 USDT x BTC, this will be executed when you get orders for USDT x BTC exchange with the same or lower value (8999, 7890, etc.).