Make The Text Bold
This button in a "toggle" button, this means that if it is depressed (looks like it is pushed in) the text will be bold, and if it isn't despressed, the text will not be bold. If you click on this button when it is depressed, the button will no longer be depressed. If you click on this button when it is not depressed, it will become depressed.
If you have selected some text, depressing this button will make the text bold .
If you have not selected any text, depressing this button will make the text that you type after clicking on this button bold text.
The bold is circled in the picture below, in this picture the bold button is depressed.