Unix systems (and thus Linux) are multi-tasking and multi-user systems. Indeed, several processes can run in parallel, and be owned by different users: the kernel mediates access to the resources between the different processes. As a part of this task, it has a concept of priority, which allows it to favor certain processes over others, as needed. When you know that a process can run in low priority, you can indicate so by running it with nice program. The program will then have a smaller share of the CPU, and will have a smaller impact on other running processes. Of course, if no other processes needs to run, the program will not be artificially held back.

nice works with levels of “niceness”: the positive levels (from 1 to 19) progressively lower the priority, while the negative levels (from -1 to -20) will increase it — but only root can use these negative levels. Unless otherwise indicated (see the nice(1) manual page), nice increases the current level by 10.

If you discover that an already running task should have been started with nice it is not too late to fix it; the renice command changes the priority of an already running process, in either direction (but reducing the “niceness” of a process is reserved to the root user).