Buy the ask & Sell the bid

Every tradable stock or option has two prices quoted at once: the bid, which is the highest price someone is currently willing to pay for it, and the ask (also called the offer), which is the lowest price someone is currently willing to sell it for. "Buy the ask and sell the bid" describes taking whichever side gets you filled right now instead of waiting for a better price.

If you want to buy immediately, you pay the ask, because that's the price sellers are already offering. If you want to sell immediately, you accept the bid, because that's the price buyers are already offering. This is exactly what happens when you place a market order: it doesn't specify a price, it just says "fill me now," so it crosses the spread and takes whatever the other side is asking or bidding.

The gap between the two prices is called the spread, and it is the cost of demanding immediacy. A stock with a tight spread, a penny or two, costs you almost nothing to buy the ask and sell the bid. A stock with a wide spread, common in thinly traded stocks or many options, can cost you real money the instant you enter, before the price has even moved. That spread is generally captured by whoever was already standing on the other side of the trade with a resting limit order, not by a single "specialist" figure — on modern electronic markets it's usually other traders, market makers, or algorithms providing liquidity, not one designated person or firm.

The nuance beginners miss is that buying the ask and selling the bid is not a strategy, it's a description of what a market order does. People say it to mean "I got in/out aggressively" as opposed to placing a limit order and waiting for the market to come to their price. Doing this repeatedly on wide-spread instruments quietly erodes returns even when your read on direction is correct.