“Multimedia includes a combination of text, audio, still images, animation, video, and interactivity content forms” – Wikipedia
To me, multimedia is all about using media in new ways in the modern age of computing and the internet. Multimedia takes advantage of the characteristics of the medium of personal computers and other devices and uses it to enrich presentations and demos with audio, video and other interactive content to make interaction with users and customers more engaging, memorable and entertaining. Whereas in the past a business would have had a brochure, catalog and live demonstrations, now businesses can have a highly-engaging web presence, use extremely polished presentations, manipulate complex data and present it in amazing new ways that is meaningful to an audience rather than presenting raw numbers, to name just a few ways multimedia is impacting businesses.
See this video for a demonstration of that last example