A user is entering text into a text box. The user stops typing for a period of time, tabs or clicks out of the text box, or is otherwise done using the text box. When the user is done, server side validation is done (such as checking for uniqueness of a username on a sign-up form).
Attach a set of events to the text box that capture when the user is making a change, or leaving the text box. Every time the event fires, create a function that contains the functionality you want, to be fired on a timeout. If the event is fired again, cancel the existing function.
For more details on the plugin, visit its GitHub page.