It is not a new unit of Bitcoin. NAV refers to Net Asset Value. mNAV is a premium (or discount) of the NAV vs Bitcoin held.

Take the value of the company (for bitcoin treasury companies like Microstrategy, you can use the market cap).

Divide that value by the fiat value of the bitcoin held.

Overtime this will trend towards 1.00. When its above 1, people are paying a premium to acquire that security. This is common for cashflow positive companies and those aggressively performing investment raises to acquire Bitcoin. When its below 1, that means people are paying less then the underlying Bitcoin is worth. This may occur in significant market downturns where value has overcorrected to the downside and represents opportunity