Like a cross section of a tree trunk, each ring on this graph represents one year. The data being represented is the monthly unemployment rate in the United States from 1948 to 2016. The bluer the ring (or section of a ring), the higher the unemployment was. You can hover over any section (month) of the graph to get more detail.
This chart crams a ton of information into an easily understandable graphic and uses the cyclical nature (years) of the data efficiently.