Make The Text Italic
This button is a toggle button, just like the bold button, in that help article, toggle button are explained.
If you have selected some text, depressing this button will make that text italic. Clicking on this button so it is not depressd will make the text un-italisized.
If you don't select any text and you depress this button, the text you type after this will be italisized. If you click it again before typing any text, the next text that you type will not be italisized. If you un-depress this button after typing some text, the text that yo uhave already typed will remain italisized and the text that you type after this will not be italisized.
The Italics button looks like the button that is circled in the following picture. It is not depressed in this picture.