A heatmap is a visualisation of user behaviour on a page: colours show where people click, how far they scroll and where the cursor lingers. Red areas are hot spots of attention; blue ones are ignored.
Heatmaps answer questions numeric analytics can't: do people even reach the pricing section? Do they click an element that isn't a link (and get frustrated)? Does a key button vanish under the thumb on mobile?
Three basic types: click maps, scroll maps (how many users reach which depth) and cursor-movement maps. Session recordings complement them – videos of anonymised visits, priceless for diagnosing form abandonment.
In practice: a heatmap generates hypotheses, not verdicts. Nobody scrolls past the hero? Hypothesis: the page looks finished. A test will decide.