Why do menu items blink twice when selected? I wanted to know why while clicking on an written item (quit, hide, option...) the item in question blink twice very fast and then do the action we wanted?

Thanks
It is simply a visual cue that you have selected that menu item.
It is described in Apple's &quot;OS X Human Interface Guidelines&quot; under Menu Behavior (page 134):

When the user chooses a menu item, it blinks briefly to confirm the
user’s choice and then performs the action.

(Note that this document has been retired and is no longer available for download, but a copy is available at https://issuu.com/hyungjoonjun/docs/osxhiguidelines)
OS X provides other visual cues: when you press a key shortcut, for example ⌘+C, the menu bar menu where that key shortcut is listed, in our example Edit:

is highlighted for a fraction of a second.