It moves around too much to be the amount of disk used for stored objects. It moves around too little to be the size of swapped-out part of squid. Median time to service an HTTP request in the last 5 minutes. Unit is probably milliseconds. It jumps around too much for that.
Unit is probably bytes.
Very useful if your cache is slow and you don't know why. Does not work. Same as the above. Such things as number of ftpget processes are understandably missing, since ftpget is no longer external in squid 2. The object and volume hit rate can be calculated with some effort.