NAV is the "unit price" you actually transact at with a fund: take the value of everything the fund owns — stocks, sukuk, cash — subtract its liabilities such as accrued expenses, and divide by the number of units outstanding.
In open-ended funds (most retail funds), you subscribe and redeem at the NAV computed on the valuation days set in the fund's terms — daily or twice weekly, for example. There is no counterparty to haggle with; the fund itself issues and redeems units at this price.
The most common misunderstanding: a low unit price does not make a fund "cheap", nor a high one "expensive". A fund priced at SAR 10 per unit is not a better deal than one at SAR 100 — what matters is the percentage change in price, not its absolute level, which merely reflects the fund's history and unit count.
Exchange-traded funds (ETFs and REITs) show two prices: the live market price and the published NAV, and the two can diverge — that gap is itself information investors read.