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

Realized Capitalization is a measure of combined USD values of all coins 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 Bitcoin owners.

Market Cap crossing under Realized Cap signifies life-time investment opportunity as the entire market on aggregate goes into an unrealized loss territory which happens extremely rarely.