Breadth is the simplest honest number in an index report: of all the members, how many went up. When the S&P 500 gains 0.7% with 400 of 503 members higher, the market rose. When it gains 0.7% with 220 members higher, a handful of large stocks rose and carried everyone else's index with them; the median stock had a different day than the headline says.
That second case is common in a cap-weighted index, where the largest names can outweigh hundreds of small ones combined. Breadth is the antidote to being misled by the headline: it tells you whether a move was an advance or a hoisting.
Breadth pairs naturally with concentration, its mirror: breadth counts participants, concentration measures how much of the move the biggest contributors accounted for. A broad advance shows high breadth and low concentration. A narrow rally shows weak breadth and high concentration, and it is worth knowing which one you are looking at before drawing conclusions about the market.
WMTMT reports breadth for every index, every session, alongside the full point attribution that shows exactly who did the lifting.