In Arabic, ال ("al") is equivalent to "the" in English. It is prefixed to the noun.
كِتَاب = a book (indefinite)
الكِتَاب = the book (definite)
When al- precedes certain letters (sun letters), the lam of al- is assimilated into the first letter of the word.
Sun letters: ت ث د ذ ر ز س ش ص ض ط ظ ل ن
الشمس = ash-shams (NOT al-shams)
النور = an-noor (NOT al-noor)
The lam is clearly pronounced.
Moon letters: ا ب ج ح خ ع غ ف ق ك م ه و ي
القمر = al-qamar
البيت = al-bayt