Transmedia storytelling is when multiple stories are told through a number of medium that all fit together to tell a story that is spread widely throughout its audiences.
To simplify this idea, you could use the example of the well-known sequels such as the Harry Potter series or The Hunger Games series. Both were originally a book series, they were then remade as movies, and now they are spread over a number of platforms, including video games, audio books, apps, comic books etc. With each medium telling the multiple stories, the original of the story broadens the stories audience.
Another example of this is the Twilight saga. The original Twilight movie had a following 4 movies to continue the story. Each had their own story lines, but all fit together to tell the one main story – these also being told over a number of platforms.
On the topic of Twilight, I attempted to create a SoundCloud, however had no idea how to use the platform, so I created an audacity clip instead.