This is the 2nd most frequent Azerbaijani word.
bu
"This"
The word "bu" is used as a demonstrative pronoun meaning "this," referring to the specific book belonging to the speaker.
Bu mənim kitabımdır.
This is my book.
The word "bu" is part of the temporal expression "bu gün," meaning "today," indicating the present day.
Bu gün hava çox gözəldir.
Today the weather is very nice.
The word "bu" is used as a demonstrative adjective to specify "this idea," highlighting the particular idea being referred to.