Metadata is "data about data" — in the context of digitizing things, it means information about the stuff that's been digitized. Metadata is important because it allows the information to be sorted, searched, browsed, and cataloged. Consider a directory of files called FILE1.JPG, FILE2,JPG, FILE3.JPG — you know nothing about the images in that directory. But with metadata attached, you can learn that they are all pictures of famous writers, and who took the pictures, when they were taken, and where, and so on.
Metadata can be built into the file itself — for instance, a metadata format called EXIF provides the sort of picture information discussed above for image files, plus techie stuff like lens aperture and shutter speed. Similarly, MP3 audio files often include metadata about the song title, artist name, date recorded, and so on. Or the metadata can be external to the files — an index.txt file in a directory can provide the same sort of useful information about the other files in the directory.
As you digitize things — any type of data, in any format — it's important to take the time to add metadata to the stuff you digitize. It has been said that "metadata is a love note to the future." You've taken the time and effort to digitize things: adding metadata will let people in the future find and utilize it.