to the best

An order "at best" has the conditional statement to be compiled only and only at a specified price or better.

Unlike a normal order, a market order will expire if the specific price target is reached and / or exceeded. The percentage of trade "at best" on the market, compared to the general number of transactions, depending on the liquidity and the state's robust economy.
Order to understand better

If the price of an asset is falling, the investor will hardly be able to execute an order "at best" because buyers wait for further falls.

For example, if a share is trading at $ 10, a trader can place a market order to sell for $ 10.10, in other words, any price greater than or equal to $ 10.10 will trigger the sale.