A Sankey diagram is a way of visualizing a path along a process leading to one or more possible endings. Line thickness denotes the number of times a specific thing happened along that path. So for website log analytics if I tag an event “LoadedStartPage” at 1:00pm, then at 1:02pm I tagged “ViewedDescriptionProductX” and at 1:05 I tagged “SubmittedOrder”. I could then extrapolate from additional data around the record what factors or demographics push customers toward my ideal “SubmittedOrder” status and what paths they took to get there.
We would use it pretty often for things like the above mentioned website navigation/customer journey. But there are other options as well.