For example, looking at the number of advancing stocks compared to declining stocks on the NYSE is a breadth measurement. Or looking at the number of stocks above their 200 day moving average. Or looking at the percentage of stocks in Stage 1 and 2 configurations. In general, a technically healthy market should see a lot of stocks advancing, not just the Dow 30. In general, a divergence is said to occur when two readings are not moving generally together when they would be expected to.
Divergences can signify turning points in the market.
Divergences, like everything else, are not 100 per cent reliable. But they do provide yellow or red alerts. And the bigger the divergence, the stronger the signal. Divergence and breadth are related concepts. Commercial services aren't cheap.