What is a multiple-document interface(MDI)?

It is the parent window form which is used to launch all other related forms

A multiple document interface (MDI) is a graphical user interface in which multiple windows reside under a single parent window. Such systems often allow child windows to embed other windows inside them as well, creating complex nested hierarchies.

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Its like multi tasking the document

A user interface container that enables a user to work with more than one document at a time. E.g. Microsoft Excel.