As the compute engine processes data, it caches calculation results in case they are needed again for additional calculates. This includes meshes and variables that have been read from the database as well as the results from more complicated calculations involving expressions and operators. VisIt's compute engine periodically clears the cache of items that it no longer needs but if you want to explicitly clear the cache to free up more memory for your computer's other applications, you can click the Clear cache button in the Compute Engine Window.