This is something that the ones who have had a computer for a long time are going to assimilate much easier than newcomers. I say so because older users are more likely to have used a command line before than those who are relatively new to computers.
The latter will not necessarily be acquainted with something like that unless they are into programming.
A command line interface is basically an environment in which a user can interact with his computer just by typing commands directly. This stands in direct opposition to the GUIs (Graphical User Interfaces) that most users are familiar with nowadays.
TweetShell, then, takes this approach and brings it into the Twitterverse. It lets anybody tweet in such a context, and do so from the browser – it is a web-based application, and it can be accessed from anywhere an Internet connection is available.
What good is this for? Well, it is one of these “specialized” apps. That is, it will appeal only to elitists. The average Joe will not get near it, for the simple fact that it doesn’t make life easier. But those who have a somehow pronounced sense of independence and personality will like to give it a try. And of course, those who are known to swim against the tide at each and every turn will be in their element here.