The two main concepts in the trading of securities, stocks, currency, or other investments are the bid and ask prices, which represent the prices for buying or selling a security.

The bid price is the highest price a market participant or buyer is willing to pay for a security. This price is usually based on the current supply and demand for the security in question. A high bid price is indicative of greater demand for the security and thus, higher liquidity for its market.

On the other side of the spectrum, the ask price is the lowest amount a seller will accept for a security. A low ask price may represent a desire by a seller to quickly offload an investment, or a lack of overall demand for a security, leading to a decrease in its liquidity.

The spread in the market is the difference between the bid and the ask price, and is often used to measure the liquidity of a security. A security with a low spread indicates a larger demand and greater liquidity, while a higher spread suggests there is more supply than demand, and thus less liquidity.

In summary, the bid and ask prices represent the supply and demand of a given security, and the spread between these two prices indicates the liquidity of the security. Investors often use this spread to determine the desirability of a particular security in the market.