When terrestrial broadcast TV (with cable being significant in some countries) was the only way of distributing video signals over large geographic areas, the result was that there were (and still are) huge audiences for a small number of channels.
The addition of satellite TV, and an increase in the number of cable channels, means that audiences are now divided between more operators and a far greater number of channels.
Similar changes are affecting other media. Digital radio and podcasts are dividing the audience for audio. Completely new media (like the web) are competing with old media, dividing the audience up further.
This is the fragmentation of the media.
The result has been not just loss of market share for media companies. It has also made advertising more complex. Advertisers need to buy and manage ad space/time over a much greater number of channels. This has created opportunities for media buying businesses.