ETL is the process in Blackboard Learn which extracts, transforms, and loads reporting data into the BBLEARN_STATS schema. It takes data from Activity_Accumulator and various other Blackboard Learn tables and processes them into ODS_XXX tables within the BBLEARN_STATS schema. The ETL process runs periodically throughout the day to keep these tables up to date. When you run Course/System Reports, the data is being pulled from these ODS_XXX tables. In order to have the latest data for a course report, the ETL process must be functioning as expected.
If you notice Course Reports don't contain up-to-date data, it could be that ETL_REFRESHDATA is not functioning properly. If you're a self-hosted a client, you may also begin to notice a performance impact from ETL/ODS related queries in the database.
When these types of issues occur, create a Support Ticket via Behind the Blackboard and together we can begin an investigation into the problem.