ETL is a concept through which applications are built, like OOPS concepts.
ETL stands for Extract Transform and Load.
Say, you have a marketing team that fetches data from different sources like Facebook, Twitter, Google AdWords, etc.
Your marketing team creates the consolidated report of how much amount is spent in total and the ROI.
Now, your marketing team is doing everything manually. Like they log in to each portal, download the data and consolidate in an Excel file. Apply some business logic and send it to the CEO.
Doing this every week or month will be the tedious task.
Now, your IT team uses a tool you have mentioned above or develops their own that takes data from each portal mentioned above like Facebook, Twitter, and Google AdWords. That is Extract.
Now you apply business logic on top of it, that is transform.
You load this data through BI tools. i.e. Load.
So, as a tester, you should make sure that the tool or software you are using is good enough to move data from one place to another.
You should be checking Performance of DB when lots of data is loaded.
Make sure that all data are transformed properly and all business logic is in place.
So, you can use the same tool from above list to build your test suite.