Create a primary Menu
You will setup menu for the theme in Dashboard > Appearances > Menus . To create a menu, just click add new menu choose the menu items from the left hand side, add theme to Menu, you can modify the lable of them, change the order and make theme be parent-child by dragging the items forward to the right a bit. Remember to check the box at the bottom to decide the location for the menu you've created
Finally click Save to finish before you go somewhere else or start to make the next menu
Add description to primary Menu
To add description to menu, go to Appearance -> Menu, select Screen Option at the top of page.
Then check the description box.
You will find option to add the description here!