These commands used to set up the priority of processes in any way the user like to do. If you want to set high priority you can do it by changing the nice value with the help of Nice command & if a user want to change a nice value of a running process, then he/she have to use Renice command.
Note :- Only a root user can set priority high by changing the value in negative form and a normal user can change a process to low priority only.