The index itself is built from six components: household income, poverty rate, house value, contract rent, share of adults with a bachelor's degree or higher, and share of white-collar employment. The six components are combined into a single score, which is then benchmarked against its own metro region rather than a national average, that's what makes the index meaningful.
Clicking or hovering a tract opens a detail view breaking the score back down into those six underlying indicators, alongside the CBSA it belongs to. The goal was to let the map work at two zoom levels of understanding at once: a color gradient for pattern-spotting across a whole region, and a per-tract readout for anyone who wants to know why a specific place scored the way it did.