When it is used with the definite article (al-Rabb) the Arabic word denotes "the Lord (God)".
In other cases, the context makes it clear as to whom the word is referring to, in this case, "rabb" refers to "owner, master", for example rabb al-dar (رَبُّ ٱلْدَّار) means "master of the house/residence".
God in Islam is referred to by many qualities and attributes.
In the first Surah, Al-Fatihah of the Quran, introduces this Title "Rabb" in the first Verse, "All Praise and Gratitude is due to God, Rabb of all the worlds and Universe", thus stating clearly that God takes care, nourishes, fosters through every stage of existence, in which everything between that exists.
In the Indo-Gangetic plain, especially in the Punjab region, the term "Rabb" or "Rab" is used by Muslims, Sikhs, Hindus and Christians to refer to God.