The Heatmap provides a way to see the entire distribution of a metric's values, in a compact format. In the Heatmap view, each column corresponds to a specific time band. In the 24 hour view, each column corresponds to 1 hour of data. In the 7 day view, each column corresponds to 8 hours of data.
The Y axis respresents a set of buckets - each each row corresponding to a specific 500 millisecond bucket. So and value at the 3 bucket (the 3rd column from the bottom) holds counts for experiences between 1000-1500 milliseconds.
The actual data if both shaded (with higher counts yielding darker contrast) and scales with the Y axis. Values higher in the Y axis (e.g. slower experiences) will render more red - eventually ending at the most red when the metrics's value exceeds 7000 milliseconds (7 seconds).