The trace view shows the results of an engine processing of a single flow and it appears in many places throughout: in the fiddle, when editing something etc.
You see the tree of message decomposition and execution. At each place you see the values etc.
Hovering on the colorful labels will often give you more details.
Also, if a hand cursor appears while hovering over some of the colorful labels, you can navigate to the place that message was generated etc.