What does the "nice" value mean in CPU utilization statistics

I am relatively new to Linux and Ubuntu. I have attached the system monitor to the bar (the upper bar of the screen) and opened the preference to configure this gadget. On the processor tab I see that I can mark the choose 5 colors : user, system, nice, IOwait and idle. I can understand all 4 (I think...) except for "nice". What is this "nice" ? I read this answer and I understand that the superuser can perform tasks with greater priority but , what does it mean if I see it in the system monitor without me running any new nice processes ?

The nice value for the processor is

the time the CPU has spent running users' processes that have been "niced".

(from man top) A "niced" process is one with a positive nice value. So if the processor's nice value is high, that means it is working with some low priority processes. So this indicator is useful when you see high CPU utilization and you are afraid that this high load will have bad effect on your system:

• High CPU utilization with high nice value: Nothing to worry, not so important tasks doing their job, important processes will easily get CPU time if they need. This situation is not a real bottleneck.
• High CPU utilization with low nice value: Something to worry because the CPU is stressed with important processes so these or new processes will have to wait. This situation is a real bottleneck.

I think this nice value is not too important for a simple user, this is more useful in a server environment.

The nice field in the CPU bar measure those process that are nicer to other process. The CPU priority in Linux are expressed in nice values, between -20 and 20, the higher the nice value, the lowest the priority in the CPU.

What you are seeing is a nice (low priority) process working and using the CPU. This is to prevent you from thinking that your system is under load that would happen if you only see 100% utilization and indicating that when you need those cycles you will have them.