Every process in UNIX runs with a particular priority assigned to it, which determines how much processing time should it be allowed to use. Priorities are represented as numbers from -20 to 19 where -20 represents the highest priority, because -20 comes before every other number on that scale, and 19 is actually the lowest priority because it comes last.
This is probably part of why this value is called the "nice value", because the greater the number the "nicer" is the process when it comes to demanding resources. A process with the value of 19 is the nicest because it is the least selfish, so to speak.
The nice command allows you to start a process with a particular nice value, setting the process priority, whereas the renice command allows changing the nice value of an already running process.
To start a process with a custom nice value you run the desired command prefixed by the appropriate nice command, like this:
nice -n 10 znc
So the -n option says we want to adjust the nice value, the 10 is the actual value, and znc is the command in this example. This will start the znc process with the nice value of 10, a fairly low priority.
Now if you use the ps command, which lists running processes, to search for a znc process you'll see that its nice (NI) value is 10:
ps -l -C "znc"
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
1 S 1008 13631 1 0 90 10 - 5251 - ? 00:15:05 znc
The -l option tells it to show the process in the long format, listing all information about it, and the -C option followed by the command just tells it to look for a process started by that command.
If you've got an already running process you can change its nice value, and therefore its priority, without restarting it. Simply run the renice command like this:
renice -n 7 -p 13631
So the syntax is the same except instead of specifying a command we're specifying a process ID (PID), with the -p option. We need to have a process ID of the running process before we can renice it. You can find it in various ways, but the simplest is to just use the above mentioned ps command:
ps -l -C "znc"
Then simply look for the PID field for the value to put after -p and you're set.
When you run the ps command after renicing you'll see the new NI value reflected.