Rabb has three meanings: (i) Lord and Master; (ii) Sustainer, Provider, Supporter, Nourisher and Guardian, and (iii)Sovereign and Ruler, He who controls and directs. God is Rabb in all the three meanings of the term. The rational of the basic Qur'anic message - 'serve none but God' - is that since God is man's Rabb - Lord, Sustainer, Provider, Nourisher, etc. He alone should be the object of man's worship and service.See, for example, Qur'an 2:21.