A Cumulative Flow Diagram (CFD) is an area chart that shows the various statuses of work items for a particular time interval. The horizontal x-axis in a CFD indicates time, and the vertical y-axis indicates cards (issues). Each coloured area of the chart equates to a workflow status (i.e. a column on your board).
A CFD can be useful for identifying bottlenecks. If your chart contains an area that is widening vertically over time, the column that equates to the widening area will generally be a bottleneck.
See Viewing the Cumulative Flow Diagram