The bid price is the maximum offer available for a particular commodity at the present time. The ask price is the minimum asking price available for a particular commodity at the present time. More simply, if you want to buy, you will pay the ask price. If you want to sell, you will receive the bid price.
The difference between the two is referred to as the “bid-ask spread”, and often is a reliable indicator of an investment’s liquidity. The smaller the bid-ask spread is, the more liquid a commodity and the less “transaction fees” an investor will incur when getting into and out of investment positions.