The Text tab is only used with Source records. It allows you to add and edit the text of Text from Source for a Source record. If you want to edit the text in the Note Window, double-click on the text, or click on the grey bar just just to the right of the vertical scrollbar.
When you click on the button on the toolbar,you will be given the opportunity to select from a number of Text from Source autotexts, that will form the first draft of the new field. See Autotext to learn more.