in Delphi TIPS
Dialog windows you use to display critical information to the user are, in most cases, displayed modally. A modal form is one where the application can't continue to run until the modal (information) form is closed. Delphi's ShowMessage, InputBox and MessageDlg, for example, display a modal form to the user waiting for some action.
To emphasize the importance of a modal form and the information it presents, you could gray out the main form of the application when the modal form is activated.
Read the full article to learn how to Dim Out the Main Form of an Application When Modal Form is Displayed.