This is a price instruction on a buy or sell order. With a sell limit order, the customer is instructing that the sale be made at or above the specified price. With a buy limit order, the customer is instructing the purchase be made at or below the specified price.
When you ask a broker to buy or sell shares you can set a limit price. This will represent a maximum price you want to pay, if you are buying, or the minimum you would like to receive, if you are selling. It's best to check with your broker how they handle limit prices. For example, some will cancel your trade after a certain time if they can't do better than your limit price.