Playlist files are either .m3u or .m3u8 (with .m3u8 playlists typically coming from a Mac). A playlist generated on your computer won't work if you copy that playlist file to your phone. There are two problems, first you need to have those mp3 files stored on your phone and second you need to generate a new playlist on your phone. A playlist is just a text file, they don't contain any audio content at all, just plain text. That's why they're so small in file size, a typical playlist listing a few dozen songs is only 10 kilobytes or so. So the music files themselves have to be stored somewhere on your phone. Also a playlist file also documents the location, where they are stored on your phone. The file hierarchy on a Windows PC, a Linux PC, a Mac, and an Android phone are all different from each other, which means things like music files are all stored in different directories. Also most people are the same, they prefer to save things in different folders. So the exact same mp3 file you copied from your PC to your phone will require two different playlists because even though it's the same file, the location where the file itself is stored is different.
I used this basic 'My Playlist Maker' app from the Play Store to generate a m3u playlist file. Here's one entry I cut and copied from the playlist file (again it's just a text file so any text editor will work):
/storage/extSdCard/null/ChicagoBluesFest1990_RuthBrown.ogg
the first part '/storage/extSdCard/null' is the directory part showing the 'null' folder on my microSD card where the audio files themselves are stored. The last part is the audio file. So when a media player loads up a playlist file, it's getting the file path of directories on the device itself, and the file names of the files to play. If I were to change the folder name of that 'null' folder to 'not-null' that would make the playlist unusable because all the music files are still listed as being in the folder from its previous name. I could just edit each entry in the playlist file to so it would be '/storage/extSdCard/not-null' using a text editor, or just as easily use the My Playlist Maker app to just make a new m3u playlist file.
Also, if I were to just leave the folder name alone but change the name of that particular .ogg file, the playlist is for the most part still OK, it's just a matter of the songs in the playlist will play except that one song will be skipped as its name is now different.
So once you wrap your head around what a m3u playlist file is and what's inside, it's a lot easier to understand how they work and what happens when they don't.
