What is a metapackage? Basically, there are some packages which, although they aren't dependent on each other to function, logically belong together. For instance, the different desktops for Ubuntu, Kubuntu and Xubuntu are available as metapackages, each consisting of all the applications that make up the desktop, as well as the desktop itself. The Ubuntu one includes the Firefox web browser, Pidgin IM client and so on.
In theory, it is possible to install all of these separately, but this would be rather tiresome - in Synaptic, you'd have to pick out all of the applications individually and select them to be installed, on the command line you'd have to type out the name of every package. Apt-get would only pick up on the dependencies, leaving you to select everything else - you'd quickly lose the will to live!
That's why metapackages exist. They make your life easier by allowing you to install one package that contains all the other packages. This way, you can easily install lots of packages which are logically grouped in one go.
Don't worry too much if you're struggling to understand this at the moment. We'll be using several metapackages in my next few posts, as they allow you to solve a number of common problems that newbies have with Linux, and I'm sure all will become clear at that point!