A fund’s net asset value (NAV), refers to the underlying value of its holdings if they were all to be immediately sold, usually divided by the number of shares in circulation.
So if a ETF held 10 shares each worth £10 each, and a further 10 shares worth £20 each then the NAV of the ETF would be (£10 x 10) + (£20 x 10) / 20 = £15.
It’s a term which has long been associated with mutual funds, and is now applied to exchange traded funds (ETFs) as a measure of whether their trading price is at a premium or a discount to the value of their underlying constituents.