what is X11 and what does it do is it worth downloading i've just got a new Macbook air

Not really.

Sure. You will find all you want to know and more at XQuartz - Wikipedia, the free encyclopedia. XQuartz has replaced X11 which is no longer included with OS X.

In short, think of it as a graphical interface (e.g. window, menus, etc.) that can work across multiple systems. Applications written to use an X11 interface can run on any OS (Mac, Linux, Unix, Windows) with a suitable X11 server (such as XQuartz, mentioned above).

The pros are that such an app can run anywhere. The cons being that it's kind of a lowest-common-denominator effect so it doesn't look like most native applications.

One cool effect, though, is the remote nature of X11 - it's possible to run an application on a remote machine (over the LAN, or even over the internet) and have it display its output on your local X11 system, so your mouse clicks, keystrokes, etc. are transmitted over the wire, processed by the server, and the results displayed on your screen.

As to whether it's worth it - there's no harm in downloading and installing it - it won't do anything unless any app you're running calls for X11, and any application that needs it will tell you so.

X11 is a very basic windowing system for use with the underlying Darwin component of OS X. In essence it's a windowing system for the BSD Unix that Darwin is based on.