Data level comparisons are comparisons where fields from two different data sets are placed on the same mesh and compared (typically by looking at their difference). In VisIt, this functionality is exposed with "cross mesh field evaluations" or CMFEs. These CMFEs are exposed through the expression interface.
The easiest way to set up a CMFE is through the wizard in the GUI. Go to Controls->Data Level Comparisons. This wizard will set up CMFE expressions. The pages below describe what the expressions do and how to set them up manually.
Here are some links that describe CMFEs in more depth: