In Arabic, the article "Al-" or as in Arabic ال, indicates that the name which comes right after it is "known" and "definite". Close to "the" in English or "Le/La/Les" in French.
The word Rehman, itself, is an adjective, meaning "Gracious" , "Kind" , "Compassionate". But in Islamic context, Al-rehman (الرحمن) means Allah, as in بسم الله الرحمن الرحیم. Or as ...
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Rahman (Arabic: رحمن, Raḥmān) is an Arabic male name meaning
Gracious. With nisba (Arabic onomastic), the name becomes Rahmani,
means "descendant of the gracious one" and is used as a surname. In
Islam, Ar-Rahman (The Most Gracious) is one of the Names of God.