A limit order is an order that you place on the exchange order book with a specific limit price. So when you place a limit order, the trade will only be executed if the market price reaches your limit price (or better). Therefore, you may use limit orders to buy at a lower price or to sell at a higher price than the current market price.
Unlike market orders, where trades are executed instantly at the current market price, limit orders are placed on the order book and are not executed immediately.
When should you use it?
You should use limit orders when you don't want to buy or sell immediately, but at a certain price instead. Unlike market orders, the limit orders are not executed instantly, so you need to wait until your ask/bid price is reached. Limit orders allow you to get better selling and buying prices and they are usually placed on major support and resistance levels.
How does CoinPanel handle limit orders?
Placing a limit order on CoinPanel will immediately place a limit order on the exchange order book. So it is just like placing a limit order on the exchange.