Command buttons in each group carry out a command or display a menu of commands.
Each of this group in the tab has some buttons that represent some commands that you may want to use in that group. In Page Layout, the Page Setup group has buttons like Margins, Orientation and so on. As you move your mouse over them, a small description of these commands is shown.

2
Click on Home tab  
Dialog Box Launchers are small icons that appear in some groups in the bottom-right corner.
Clicking a Dialog Box Launcher opens a related dialog box or task pane, providing more options related to that group

3

Click on the small icon - Dialog Box Launcher in the Font group in the Home tab

Font dialog box will open