"Market Cap" / "Realized Cap" = (sum_(i=1)^n "CoinValue"_i * "Price"_"now") / (sum_(i=1)^n "CoinValue"_i * "Price"_"at creation")`
MVRV is a ratio between Market Value and Realized Value of a set of coins. It's one of the metrics in an Unrealized Profit/Loss category of metrics.

Market Value is a measure of the current valuation of an asset i.e. a combined value of the entire coin set in USD terms.

Realized Value is a measure of combined USD values of all coins in a set at the points when they were received (roughly, purchased) from a previous owner. It can be thought of as a collective cost basis for all the owners of coins in the set.

Naturally the higher MVRV value is above 1 the more unrealized profits the coin owners are sitting on at any point in time and the bigger is the incentive for them to sell.

MVRV values below 1 signify pain for the coin owners as collectively USD values of their holdings dips below the level they 'purchased' a coin(s) for.

The metric was introduced by Murad Mahmudov and David Puell in this article.