“ال” (al)the Arabic equivalent of “the.” It makes the following noun specific.Corpus grammarDET — prefixed definite article.
“رَّحْمَٰنُ” (rraḥmānu)means “The Most Gracious” in this verse. It is an adjective. a describing word. Here it is masculine — singular, nominative.Corpus grammarADJ — masculine singular nominative adjective.