When you’re playing music in iTunes, the iTunes LCD – the section at the top of the window that shows the name of the track you’re listening to, and a progress bar – shows the time of the current track. At the left, it shows the full time, and at the right, you can display either the elapsed time or the remaining time. This isn’t usually important when you’re listening to music, but you might want to know one of these numbers when listening to, say, a podcast, if you’re searching for a specific point in the file.
You can toggle the information shown at the right; just click on it. It changes from elapsed time to time remaining (or vice versa, if the time remaining is visible first):
You can also toggle the information in the MiniPlayer window. It shows one of three numbers, at the right of the window: either the full time, the elapsed time, or the remaining time. Just click the time to toggle through those numbers.