Playlist should show the FID of the current playlist
FID only shows something if you put notify=1 into your config.ini [output] section. Once you've done that then it will be what the FID of the current song is.
Cac is the size of the Linux VM page cache.
Buf is the number of pages used as buffers.
Fre is the number of pages free.
The first number for load average is for the last minute, then it's the last 5 minutes and finally for the last 15 minutes.

- Trevor