Metadata, simply put, is information
about a file. It may or may not be important to the application that's using it, but it often has meaning to the user. For example, the photos in your iPhoto library (unless it's full of images you saved from web pages) all "know" what model of camera took them, because that info is part of their metadata. iPhoto might not give a (insert expletive of choice) about what camera you used, but it might help you decide which of your two cameras takes better pictures.
A more general example of metadata is tagging, which just means associating keywords with something.
Leap is a sort of alternative file browser which, instead of exposing the traditional folder hierarchy to the user, organizes and displays files by their tags, which get inserted behind-the-scenes into the Spotlight comments. While, at first, such a radical departure from the structured world of folders might make traditional users like us gasp in horror, it eventually becomes clear that tags will allow us to find files based on
what they mean to us. The computer couldn't care less about what tags we assign to a file, but the computer's just there to serve us anyways.