In UI, modal dialogs (aka overlays and dialog windows) refer to small windows that appear on the top of the main content and require users to interact with them. Modal dialogs are notably interruptive, blocking the main content until users dismiss, confirm, or take the needed action.^[1]

In contrast, non-modal dialogs don't impede user flow, and users can continue doing what they are doing while the dialog stays open.

The most common use case of modal dialogs is to alert users to an error or inform them about something critical that demands their attention and action. Dialogs may appear intrusive, annoying, and interruptive but they help users detect problems or fix them before further damage is done.