Motif can be two things, either a graphical user interface guideline and development toolkit for the X Windows System. It is also the name of a Microsoft Windows program object for a sound sequence or music motif.
With regards to the GUI, it is designed to define the roles and tools for the look and feel of a graphical user interface. It is used in most UNIX based operating systems and their applications.
Used as the toolkit for the Common Desktop Environment, Motif became the standard widget toolkit for UNIX.