Two temperatures showing on boiler

What heating system do you have? Make / model will assist in being able to provide information.

I can't see what this behaviour means, it's not mentioned in the manual, but I would imagine that if you are turning the Central Heating Temperature knob, the first temperature will be the current temperature of the water, and the flashing figure the set point. If you turn the knob, only the set point temperature will change, unless the boiler is firing in which case the first temperature will rise towards the set point.

The Central Heating water temperature determines the maximum output from the radiators. Turning the temperature down on the boiler reduces the maximum output of all radiators. So you should set it as low as possible but high enough that your rooms are heated up reasonably quickly and so that radiators are not going to burn anyone touching them, especially children who burn more easily.

The Thermostatic Radiator Valves (TRVs) shut down the flow of water to the radiator when the room is hot enough, so use the TRV to limit the temperature of the room to a comfortable level. The TRVs stop each room becoming too warm, but need the heating to be on to heat to room at all.