What does 'Nice' mean on the processes tab

'nice' refers to the priority the process gets on your CPU. This is a number in the range of -20 (highest priority) to 19 (lowest). You'll probably find that most applications you're running are at nice '0', which means that they are running with normal priority and none will get preferential treatment by the scheduler over another.

If you're running an intensive program, you may wish to launch it from a terminal with nice -n 10 /path/to/program. That way it will impact less on the performance of your system.

Note that you need root (sudo) privileges to schedule a process with priority greater than normal (<0). This is to prevent users from being able to bog down the entire system easily, and so that critical tasks can always receive the CPU time they require.

There's a good explanation on Wikipedia too.

Nice is not simply priority multiplied by minus one.

You will find that on unix/linux systems that if a task is hogging the processor, then the system does not grind to a halt. This is because the system will dynamically lower this tasks priority.

The niceness influences the base priority. So while niceness can be thought of as negative priority, it will have less effect than priority would have on a Microsoft NT system, as the Linux scheduler is already doing a good job at keeping things nice.