A limit order lets you buy or sell at a fixed price that you determine, sometimes providing a better price depending on how the market moves.
The advantage of limit orders is that you know what you will get in terms of price: limit orders guarantee you won't be matched with a worse price than what you specified.
You can also use limit orders to wait for the market to reach a certain price target, rather than just taking whatever is available in the present. The disadvantage of limit orders is that there's no guarantee the order will completely fill (or fill at all).
Learn more about how to set Limit prices.
Note: Limit buy orders will only be placed on the orderbook if the limit price is below the current market price and limit sell orders will only be placed if the limit price is above the current market price. If the limit price you submit is not consistent with these conventions, the order will be executed immediately as a market order.