In case you're not familiar with the word, metadata means "data about data". In the case of MP3 files, this means all the extra columns of information that's stored inside each file and which is displayed when you view a folder full of MP3 files in Windows Explorer. Mp3 file.
There are fields for the artist, the album the track came from, the track name, the length, the bitrate or quality, and so on. All of these are in addition to the actual filename, and will vary depending on where you acquired the file from. Also, some MP3 players try to be clever by attempting to complete missing metadata by reference to online databases. So a track which didn't contain any metadata when you downloaded it, might subsequently inherit lots of data as a result of you playing it within a particular program.
Why does all this matter? Because sometimes you might want to tidy up all the metadata and associated tags in your music files, and to correct errors. Or, as happened to me recently, you might want to rename your files so that the filename is the same as the title of the track stored in its metadata.
Doing all this manually is easy but takes a long time. Thankfully, there are programs that can help you, and my favourite is mp3tag. It's free to download and use, though the author does ask for an optional donation if you find it useful. The download is at http://www.mp3tag.de/en/ and is only around 3 MB. It's also malware-free according to VirusTotal and Web of Trust.
If you have a collection of digital audio files, use mp3tag to take a look at all the extra data contained in those files. You may be surprised at just how much useful information is in there. The program even has an option to export that data, which is useful if you want to create a detailed list of all the music you have.