Each UTXO (Bitcoin coin) has two attributes associated with it - the date/time when the coin was created (age) and the price at its creation point (realised price).

This histogram packs the two attributes into a single chart. The entire universe of Bitcoin coins is broken down in one dimension into age groups (color) and in another dimension into realised price bins (x axis).

The chart is showing how much Bitcoin (y axis) was realised (purchased) at a specific price range and for how long has been sitting there dormant (color).

It can help you see the structure of the market as of today and answer questions such as how many longer/shorter term holders are in profit/loss or where the selling pressure may be coming from or how much pain certain market participants are experiencing atm.