Cat Flippen explains:

Transmedia Storytelling references the process of using various parts and elements of a topic that are shared periodically via multiple means of delivery (transmedia) for the explicit purpose of drawing in, intriguing and entertaining those who are targeted to receive the transmedia.

And gives an example using The Matrix:

Parts of the entire story were spread out and portrayed through a variety of media: three films, two comic books, many video games, and animated short films. In order to grasp the entire story, you had to partake of the entire “universe” of The Matrix. The mix of the old media (the television show, the movie) with the new media (split story experience, fan-driven participation) is seen often.