A double-click in simply a technique in which we make two clicks with the left mouse button in a very short delay (less than a second).
Not always needed
What can be confusing with double-click its that it is not always required, sometime only one click is enough to open of choose something on the screen. Most of time, double-click is necessary in a context where we can choose more than one elements at the same time ( see multiple selection with the mouse
), for instance with files and folders, email lists, etc.
Becoming less used
Double-click is less and less used in mobile devices such as ipads, android smartphones, etc. but it stays omnipresent in computers of all types.