The PopUpAnchor control is part of the Spark component set and available only in Flex4
The PopUpAnchor control displays a pop-up component in a layout. The PopUpAnchor displays the pop-up component by adding it to the PopUpManager, and then sizes and positions the pop-up component relative to itself. Because the pop-up component is managed by the PopUpManager, it appears above all other controls.
With the PopUpAnchor control, you can create various kinds of popup functionality, such as the following:
Click a button or a hyperlink, and a form to submit feedback pops up
Click a button, and a search field pops up
These first two scenarios each contain three controls: the button or hyperlink control, the PopUpAnchor control, and the component that pops up (the form or search field).
Mouse over the top of an application and a menu drops downIn a calendar tool, double-click an appointment block to open an Edit dialoge
Find out the more detailed explanation here