combi boilers provide hot water when you open a hot water tap i.e. on demand.
the timer is for the heating. timers usually have 3 settings, constant, off and auto (timed)
have you just had the boiler installed? your installer should have shown you how to set the timer or have set if for you as part of the handover process.
it should show you how to set the programmer in your instruction book. if you haven't got a book ring vailant and ask them for a copy.
im not sure about the eco setting to be honest as i haven't serviced/worked on many condensing boilers yet.