This is just a measure of how much of its own life the process has been sleeping. It is used mainly to determine how much priority to give to the process. A process that sleeps most of the time would be given a lower priority than one which sleeps very little.
cpu time is a slightly different concept. You can get more statistics out of /proc/<pid>/stat. These include things like user time, kernel time etc... If you type "man proc" and search within the man page for "time" you'll see utime etc... explained.
Also typing "man ps" would give you all the different options you can see for processes including time fields.