Nice?

'Nice' often kicks in when the processor is taxed... it starts divvying up the scarce resources to apps based on priority. Say when Spotlight is initially indexing your disk, it takes up a lot of CPU... doing anything else intensive causes the Nice % to jump up because there's too many things going on for each app to get what it wants.

I'm sure someone else can give a more technical explanation.

 

If a program is running in nice mode, it means that it takes low priority and doesn't tie up CPU time that other programs need. It's often used to run really long CPU intensive jobs to avoid hammering the other processes on the computer. For example, on our schools UNIX machines, if you're going to run a long (days long) processing job, you're supposed to run it with nice so that other users of the computers don't have problems with slow machines. Hope that help explain something.