Dialogs are modal, which means they block all interaction with the app until the user selects a dialog button. To visually reinforce their modal behavior, dialogs draw an overlay layer which partially obscures the temporarily unreachable app UI.
Note When Cancel is one of the available dialog options, apps can choose to let users dismiss the dialog by pressing the Escape key. This behavior is not built into the control but is a commonly implemented shortcut.
Flyouts and context menus are light dismiss controls, meaning that users can choose from a variety of