This is all about mp3 id tags. When you locate mp3's in your LW searches, & you hover your cursor over the top of the song, magical details appear about it's title, artist, year, etc.
iTunes has the ability to add a number of tags (just click in the particular area twice but not too quickly & you'll be able to add the info you want to.) iTunes is a little disappointing in some ways b/c it doesn't always display all the info that an mp3 has in its id tags. I find the use of other mp3 players better for displaying all the info & also for adding info. My favourite for the mac is MP3 Rage. You can add a large no. of details including album covers, etc. And an alternative player & encoder is Audion 3. Audion has more codecs than iTunes & can play some mp3's that iTunes can't.

Not everybody adds mp3 tags or adds accurate details. I find I need to fix many details up. For those who don't add tags they just put the artist & title in the mp3's file name.

There's different types & versions of id tags for mp3's. The later the version, the more the details you can add (more letters also). Some mp3 softwares around can not recognise the most recent versions of tags. So some people add tags in an intermediate tag version so most if not all people will be able to access that info.