This is the 308th most frequent Arabic word.
الأكثر
"The most" or "the majority."
In this sentence, "الأكثر" is used as a superlative to describe the highest degree of intelligence among the group.
Ahmed هو الشخص الأكثر ذكاءً في الفصل.
Ahmed is the smartest person in the class.
Here, "الأكثر" specifies the leading position of the book in terms of sales.
هذا الكتاب هو الأكثر مبيعًا هذا العام.
This book is the best-selling this year.
In this sentence, "الأكثر" emphasizes the degree of importance of the topics to be learned.