Its the way google music stores songs. It assigns each file a unique numerical filename as opposed to just storing the whole file as is. So if you upload Aerosmith -Walk This Way.mp3 Google stores it as 8972998.mp3 and transfers it to your phone as such. Also, it is stored in a cache file. If you want to find your music, grab a file manager and go to sdcard/Android/data/com.google.android.music/cache and you'll see all your mp3's listed there as a bunch of files like 1539.mp3
UNFORTUNATELY, this prevents you from using any 3rd party music players as none of the track information is visible. This is a big complaint from people.
Personally, I just use the google music app. Its fine for me and the convenience is worth it. I store all my music online with google and just select the 20-30 songs I am in the mood for this week to be "available offline". Then I set the player to show offline music only.