Spread - the difference between the Bid and Ask prices. Spread is a key indicator of the liquidity of the asset. In general, the smaller the spread, the higher the liquidity. The bid-ask spread can widen dramatically during periods of illiquidity or market turmoil, since traders will not be willing to pay a price beyond a certain threshold, and sellers may not be willing to accept prices below a certain level.
Note: Generally, the larger your order, the larger the Spread. The Dashboard widget allows you to check the Bid/Ask prices and current Spread for a particular order size: