Right now, iTunes sells individual TV episodes for either $1.99 (standard-def) or $2.99 (high-def), and those episodes are downloaded, rather than streamed. Downloading episodes takes both time and storage space, especially for HD content--one 44-minute episode in HD (hour-long dramas, for example) can total over 1GB in storage space. Those episodes can be watched on a mobile device, computer, or TV--provided your mobile device, computer, or TV set-top box is made by Apple.
The rumor says that Apple wants to offer streamed episodes, a major but overdue change for iTunes, for a reduced price of $0.99 per episode. Those rentals would follow the same rules as movie rentals on iTunes already do: Rent an episode, and you have 30 days to begin watching it, after which you have 24 hours to finish it. The strategy makes sense--TV episodes aren't like music, in that you generally do not watch them over and over, necessitating a permanent purchase.
This rumor, along with the persistent rumor of an Apple TV overhaul (with accompanying iOS interface), could indicate a new, stream-heavy direction for Apple's media empire. Of course, these streamed videos would be restricted to Apple devices, but that's the norm for Apple media, and certainly hasn't prevented iTunes and iOS from becoming major successes.
Do people want an all-you-can-eat subscription service like Netflix and Hulu Plus, or an a la carte style service like this rumored iTunes? That all remains to be seen--if indeed the rumor is true.