Underline The Text
This button is a toggle button, this type of button is explained in the Bold help article.
If you have selected some text and you depress this button, the text will become underlined. If you click the underline button again, the text will no longer be underlined.
If you have not selected any text and you depress the underline button, the text that you type after this will be underlined, if you click on the underline button again the next text you type will not be underlined.
The underline button looks like what is circled in the following picture. It is not depressed.