nice is a command that allows you to change the scheduling priority of a command from -20 the most favorable to 19 the least favorable.
nice -n NUMBER
Where NUMBER is an integer number from -20 to 19, if you do not use the -n option, the niceness will be increased in 10 by default.
sudo nice -n 4 tail -f /var/log/messages
Now if you start htop you may see that tail is running with a niceness of 4.