The fair value is a term that is widely used in accounting. For example, if a company is listed (I.e you can buy it’s stock on the stock market), the fair value is the same as the market value since the market value is a good reflection of the value that you would pay to buy the stock now or a good reflection of the money you would get if you would sell the stock now.
However, not all companies are listed and if there is not an active market (a stock exchange for example), the fair value is less straightforward to be determined. In this case, you can use the dividend discount model to get a price for your stock or you can use the capital asset pricing model to get a fair value for your stock.
You could also find similar companies that are listed and use their stock price (with the necessary adjustments made to reflect the capitalisation and the fact that your company is not listed).