The Linux niceness scale goes from -20 to 19. The lower the number the more priority that task gets.
If the niceness value is high number like 19 the task will be set to the lowest priority and the CPU will process it whenever it gets a chance. The default nice value is zero.
Checking the Priority of Running Processes
Another way you can get the nice value is use ps:
ps -o pid,comm,nice -p 555
Setting priority on new processes
nice -n 10 service mysql start
Setting Priority on Existing Processes
renice -10 -p 9527