What does the low limit do, and where should it be set on an oil fired hot water furnace?
The low limit typically shuts down the circulators when the boiler (if it has a water temperature, it's not a furnace) temperature drops too low. If the boiler is operational but tasked beyond its capacity, it will catch up and the circulators restart - if not, the house gets cold and you hopefully call for service.

Typically the boiler will specify that it should never be below 120F (if it is, corrosion is a problem) and the low limit will be set somewhat above that.

It is well worth (and often legally required and/or good for your insurance) having a professional inspection/service/adjustment done at least once a year. Understanding what your boiler does is good - adjusting it yourself is dubious, at best. If it's not working correctly, there may be parts which are not working correctly and in need of replacement, rather than self-adjustment.

As for the pressure, the pressure on a hot water boiler should not move significantly. There is an expansion tank which should be adequate for thermal expansion control, and otherwise the water in the system does not change pressure, unless you happen to have VERY sensitive gauges (you won't) on both side of your circulator pump.