This is the 21st most frequent Italian word.
su
"Su" in Italian means "on," "up," or "over," depending on the context.
In this sentence, 'su' means 'on' and indicates the physical location of the book relative to the table.
Il libro è su il tavolo.
The book is on the table.
In this sentence, 'su' means 'about' and is used to specify the subject of the article.
Ho letto un articolo su Leonardo da Vinci.
I read an article about Leonardo da Vinci.
In this sentence, 'su' means 'on' and is integral to the verb phrase that specifies the object of the action of clicking.