Market Value refers to the amount an asset can be sold for on the open market, at any given time.
If you hold 100 shares of stock ABC that you can sell on the market for $50 apiece, your holdings have a market value of $5,000. Market value does not necessarily refer only to stocks. It can be any asset that can be bought or sold on the open market. Stocks tend to have greater levels of liquidity and broad-based market participation, so it is easier to disseminate a stocks market value at any point in time. Illiquid assets can be more difficult to value, such as real estate or works of art.
The market value of an asset changes over time due to supply and demand forces. Investors hope to find stocks whose market value is well below book value, meaning the stock trades at a deep discount to book value per share.