The NAV is the dollar value of one share of a fund. It's calculated by totaling the value of all the fund's holdings plus money awaiting investment, subtracting operating expenses, and dividing by the number of outstanding shares.
A fund's NAV changes regularly, though day-to-day variations are usually small.
The NAV is the price per share an open-end mutual fund pays when you redeem, or sell back, your shares. With no-load mutual funds, the NAV and the offering price, or what you pay to buy a share, are the same. With front-load funds, the offering price is the sum of the NAV and the sales charge per share and is sometimes known as the maximum offering price (MOP).
The NAV of an exchange traded fund (ETF) or a closed-end mutual fund may be higher or lower than the market price of a share of the fund. With an ETF, though, the difference is usually quite small because of a unique mechanism that allows institutional investors to buy or redeem large blocks of shares at the NAV with in-kind baskets of the fund's stocks.