The RNR is there to provide the regular RN with trained people who they can call upon when needed. Basically, you would (at weekends and evenings) do your basic training. You would then be trained in a specific role and practice elements of that role during weekends and evenings. There would probably be a bigger camp or two for a week at a time to consolidate training.
You could then be called up under the Reserve scheme and serve along side regular RN wherever and whenever needed. So, ultimately, you could spend 6 months in a war zone doing the role you are trained for.
You REALLY need to be aware of this. You don't get to pick and choose. You go where they need you. You would be part of the Armed Forces and all that entails.