That means that whenever I download something in Safari or upload something via Cyberduck, a little translucent window pops up to tell me that the download / upload is complete. It also means that whenever I get a new email, a small window pops up to tell me that I've received new messages and their subject lines, and SpamSieve pops up a window to tell me how many of the messages received are good.
Growl is one of those things that I forget isn't a default part of OS X. Every so often it reminds me of things without being overly intrusive. Since updating to Tiger, I had missed it. I just reinstalled it, so I thought I'd pass it along to the rest of you. Make sure you check out the Extras folder for plug-ins for various programs like iTunes and Mail.app. It's free. Enjoy.