Net Asset Value (NAV) is the value of a firm's assets minus its liabilities. In older investment literatures, NAV is also commonly referred to as book value or equity value. NAV is an important concept not only for REITs investments but for other investment instruments in general.
NAV can be used to represent the value of the total equity, for the example the NAV of the REIT in entirety, or it may be divided by the number of shares outstanding held by investors. For example if a REIT with a million shares has $100 million in properties and $40 million in bank loans, its NAV can be represented is $60 million or $6 per share. There is no real consensus as to what is the optimal NAV should be for a REIT. NAV should be read in the context of other metrics including prevailing interest rates and current market conditions.