The Token NAV means the net asset value. This is the price of the token based on the value of the underlying assets. The formula for Token NAV is fund value divided by the supply - you can see this on the performance page.
The market price is different; it's essentially the price of the token based on supply and demand and market dynamics.
Discounts to NAV are observed during bear markets, for example when investors and speculators have a tendency to panic sell. Market premiums to NAV are explained by popularity, increased demand, and generally more positive market outlooks e.g. during bull runs.
Consistent and significant discounts to NAV, are, however, unlikely. This is due to the smart-contract liquidation option, available for registered and verified users. This feature is explained in a different thread.