A limit order is an order to buy or sell at a specific price or better.
A buy limit order can only be executed at the limit price or lower, and a sell limit order can only be executed at the limit price or higher. A limit order is not guaranteed to execute. By placing a limit order instead of a market order, you protect yourself from buying at a price higher or selling at a price lower than you had expected.
In volatile markets, although your limit order receives protection from getting filled at an undesirable price, due to the priority of other orders, your order may not be executed even if the market is trading at your limit or better after your order is entered. Similarly, the price may move away from your limit after your order is entered, in which case your order will not be executed.