As stated in the man page:
Users other than the super-user may only alter the priority of processes
they own, and can only monotonically increase their ``nice value'' within
the range 0 to PRIO_MAX (20). (This prevents overriding administrative
fiats.) The super-user may alter the priority of any process and set the
priority to any value in the range PRIO_MIN (-20) to PRIO_MAX. Useful
priorities are: 20 (the affected processes will run only when nothing
else in the system wants to), 0 (the ``base'' scheduling priority), any-
thing negative (to make things go very fast).
In short + (plus) -> process runs slower and - (minus) -> process runs faster
Hope this helps.