The SQL panel displays the SQL for the current query. You can directly type and modify the SQL for the query.
Editing the SQL directly enables the Apply and Reset buttons.
Reset will revert the SQL back to its original state before editing, discarding any changes.
Apply will automatically interpret the SQL displayed in this panel and apply them to the query. If you make changes on this panel and forget to apply them, you will be asked what to do if you try to start a new action or go to another tab.
Instead of directly typing SQL here, you can also press the Edit button, which launches the Expression editor. This editor assists you in selecting available functions, columns, and values to add to your query.