Bid and Ask
In both traditional and crypto exchanges, trades are settled through order books. An order book contains a list of all open sell (ASK) and buy (BID) orders, thus showing a snapshot of supply and demand in a market.
The sell orders (ASKs) represent supply because they are the price offered to buyers of the asset.
The buy-side items (BIDs) represent the demand because they are the price users are willing to pay to purchase the asset.
The Best ASK is the lowest price at which an asset is sold. The Best BID is the highest price at which an asset is asked to buy.