The net asset value (NAV) of an entity the total value its assets minus all liabilities. For a fund like an ETF, NAV is often divided by the number of shares to provide a representative “per-share” value.
The total stock market ETF VTI has a NAV of 160.00 USD at the time of this writing. This means that a share of VTI represents ownership of 160.00 USD worth of stocks. The market value of VTI (the value at which it was last traded at on the exchange) is slightly different, at 160.02 USD. A redemption mechanism is used to keep market value of an ETF close to NAV. A mutual fund may owe money to lending banks and have other pending payments which would reduce its NAV.
Investopedia's article on net asset value
Wikipedia's article on NAV