The 'nice' level is a UNIX standard way of setting the priority of jobs. The higher the 'nice' setting, the lower the priority, hence the nicer you are to the rest of the world. When a new job is launched on a remote machine, the nice level is read from the screen and used for this new job.

Processing will be a bit faster for a nice setting of 0, because other processes will get less attention. In general, you want to leave this setting on 19, because then people working on other machines will have a minimum of disturbance. In practise, we always keep the nice level on 19, because when nothing else is going on on the machine, still almost 100% of the CPU time will be taken by the processing. Therefore, the advice is to put it to the right, but putting it to the left may gain some processing speed (at the cost of angry colleagues).