This is the 119th most frequent Arabic word.
والذي
"والذي" means "and the one who" or "and he who" in English.
Used as a relative pronoun to introduce a descriptive clause about the antecedent 'the book'.
هذا هو الكتاب والذي اشتريته أمس.
This is the book which I bought yesterday.
Functions as a link between the antecedent 'the article' and its descriptive clause 'was very useful'.
قرأت المقالة والتي كانت مفيدة للغاية.
I read the article which was very useful.
Acts as a connector to describe and provide additional information about the antecedent 'this movie'.