What does NAV mean?
NAV (Net Asset Value), is the sum total of the market value of all the shares held in a portfolio, including cash, less the liabilities, divided by the total number of units outstanding. Thus, the NAV of a mutual fund unit is nothing but the 'book value' of a unit.
The formula of NAV = (Assets - Liabilities)/Number of outstanding shares
To learn more about the implications of NAV on direct plans and regular plans of a certain fund, check out this post on TradingQnA.