What is a Double Click?
A double click typically refers to pressing the left mouse button two times very quickly. A double click sometimes performs a different action compared to clicking on something only once, or clicking on it twice but after a long delay.
For example, double clicking on a folder in a file manager display its contents in the manager (i.e. makes it the current location being viewed), while double clicking on a file will launch the application associated with its file type and tell that application to open the file.
More generally, double clicking on an item in a list has the effect of "activating" it. It's typically possible to activate items on lists by pressing the Enter key as well.
Left-handed users may have the buttons of the mouse swapped, so the primary mouse button is the right mouse button. In this case, "double click" would mean pressing the right mouse button twice.
There are applications that have a function triggered by a triple click, but they are rare.