Trading screens on most exchanges show order books containing a range of bid and ask prices and sizes. Bids represent the demand for a particular (crypto)currency, while asks represent the supply. Bid (or offer) prices are the amount that buyers are willing to pay for Bitcoins right now, while ask prices are the amount that sellers want to receive for their Bitcoins right now. The bid and ask sizes concern the number of Bitcoins that can be bought or sold at the given price levels.
The highest bid price is called the best bid, and the lowest ask is called the best ask. Depending on the order type, orders will be executed at the best bid/ask quotes or once these meet predefined criteria.