When you create a table, you can add a help text to keep track of your report tables and their columns easily. You do this by clicking on the pencil above the column where you want to add a help text.
Click on "Show help text" to bring up a text box. Then click on "Change" when you have finished editing your column and written your text.
When you have written your help text, a question mark will appear next to the column with the text you wrote.