In Arabic, ال can be added to the word to denote the definite article, as in صباح الخير, which literally means morning of [the] good.
A note on Grammar: In Arabic, there are two types of letters that can follow the definite article: sun letters and moon letters.
Sun letters: We use our tongue and palette to double the sound of the [sun] letter that follows the definite article. In this case, we do not pronounce the ل in the definite article ال. It is called a liaison. Example: النّور. Remember that the doubling is sounds in Arabic is denoted by the shadda ( ّ).
Moon letters: We use our [lips] or throat instead to pronounce the ل fully in the definite article ال. These are called moon letters. Example: الخير.
Listen to the audios below to better understand the difference in pronunciation between the two.
is a sun letter السْلام: س
is a sun letter الصّباح: ص
is a sun letter النّور: ن
is a moon letter الخير: خ