You are most likely running something that is nice in the background. This isn't something you should worry about under normal circumstances.
Nice is some sort of priority system for the CPU (on *nix systems). For example, if a program has a high level of this niceness and takes 75% CPU power, it will consume those cycles normally. But if another process is started and requires 50% CPU power, the latter process will have priority and thus get all the cycles it needs.
An example of this are those @home applications (seti@home, folding@home) that work in the background, but I don't know if they utilize the actual niceness system or some other thing.
...I think.