title: What do the blue blocks in spark stage DAG visualisation UI mean?
body: <p>in the following snip for the application UI, what do the blue blocks in each stage represent? </p>
  
  <p>What do "Exchange" and "WholeStageCodeGen", etc mean? </p>
  
  <p>Where can I find a resource to interpret what spark is doing here?</p>
  
  <p>Many thanks</p>
  
  <p><a href="https://i.stack.imgur.com/znTrx.png" rel="noreferrer"><img src="https://i.stack.imgur.com/znTrx.png" alt="What are the blue blocks? What do their names represent?"></a></p>