Modal dialogs interrupt users and demand an action. They are appropriate when user’s attention needs to be directed toward important information.
To better understand the difference between modal and nonmodal dialogs, let’s look at what the terms “dialog” and “modal,” mean.
A dialog (or dialogue) refers to a conversation between two people. In user interfaces, a dialog is a “conversation” between the system and the user, and often requests information or an action from the user.
User-interface modes are special states in which the same system has somewhat different user interfaces. Each mode may come with different commands, or the same command (or action) can produce different results depending on the mode of system. In other words, in different modes, the same input will have different results. For example, a computer with Caps Lock turned on is in a special mode: all the letters being typed will be shown as capital letters. Typing a letter has a different effect when the Caps Lock is on versus when it’s off. Or, in Microsoft Word’s Track Changes mode, all previously made edits and comments are visible (whereas in the normal, default mode they are not tracked or displayed).
With this understanding of “mode” and “dialog”, we can easily define modal dialogs.
Definition: A modal dialog is a dialog that appears on top of the main content and moves the system into a special mode requiring user interaction. This dialog disables the main content until the user explicitly interacts with the modal dialog.
Complete article: Modal & Nonmodal Dialogs: When (& When Not) to Use Them