This is the 489th most frequent Arabic word.
الـ
"The" (definite article).
Here, "الـ" is used as the definite article, similar to "the" in English, indicating a specific book.
الكتاب على الطاولة.
The book is on the table.
In this sentence, "الـ" is also used as a definite article, specifying a particular sky, generally referring to the sky in view.
السماء صافية اليوم.
The sky is clear today.
"الـ" assigns definiteness to the noun, making it clear which student is being referred to—it could imply a previously mentioned or known student.