The Coverage mode integrates data from the static analysis database, tcov, and gprof to show how thoroughly your software has been exercised. One use of the Coverage mode is to analyze where your software might need further testing.
This particular view shows coverage data within the context of the calling hierarchy. Each function (cube) is color-coded, indicating the portion of its in-line code segments - blocks - which have executed. The lines represent all the function calls which are statically present in the code. Those calls which were actually made are shown by the green lines. 