The Bid-Ask Spread is the difference between an offer made on a security and the price a seller is willing to accept.
The Bid-Ask Spread is the amount by which the ask price exceeds the bid. For example, if the bid price is $50 and the ask price is $51 then the "bid-ask spread" is $1.
The larger the bid-ask spread, the less liquid the market for that particular security - buyers and sellers are too far apart for trades to occur easily. When trading, investors have to pay attention to the bid-ask spread, because it is ultimately an additional cost to investing in or trading stocks.