Understand A Baseline
A baseline is your original. It is the base where all the variations are related to. Convert Experiment compares all your variations against this baseline.
By default the original is the baseline but you can change this in the report.
You might want to change the baseline in scenarios where you had an original page without a widget. You installed a widget in your CMS system to see if it increased conversion rate. Now your original has the widget, but your goal was to test the increase.
In this case you make a new variation where you remove the widget. Change the baseline so the variation becomes the original, since the variation is now the page as it was before you installed the widget.
This way we can still see if the baseline page (the one without the widget) improved with the variation (the one with the widget).