Every Apple computer (or at least copy of OS/X) comes with a built-in programming language called Applescript. So called "scripting languages" are a feature of many operating systems, and intended to automate routine system tasks. In Microsoft-world, they are traditionally called "batch files". Typically, things that a computer user can do manually, such as copying, moving, renaming files etc. can be automated with scripts.
In an iTunes context, Applescript can be used to automate many "music collection management" tasks. For example, when I download music as .wav files from Magnatune and import them into iTunes, each track is typically given a name that includes track number, track title, and artist name, something like this:
01-Some Track Title-Some Band Name
Classical tracks can be more complex, including
BWV numbers for J.S.Bach, for example. In the absence of a script, one would have to select the track or tracks in iTunes, press ⌘I to display the track info, and edit the fields manually to put the track number, band name etc. in the correct fields, before removing the surplus info from the track name. This is not a big deal for one, or even a few tracks, but Trevor Pinnock's
Les Cyclopes, for example, has twenty-two tracks. Suitable scripts can automate the whole procedure, parsing the track name, extracting the relevant parts and writing them to the correct fields, cleaning up the track name, and so on. Select all the tracks in an album, run the scripts, and you're done.
Disclaimer: I work in the computer industry, so what seems cool and easy to me could appear arcane and pointless to muggles, but I hope this explains a bit better.