Index of expression over table row or column

I am reading this documentation[1], and it has a sentence that I don’t quite understand: "The index columns (key values) can be either simple columns of the underlying table or expressions over the table rows.”, I am thinking that for the index of expressions, aren’t those expressions over table column? e.g., “CREATE INDEX idx_lower_last_name ON users(LOWER(last_name))”, “last_name" is a column rather than a row.

[1] https://www.postgresql.org/docs/17/index-api.html#INDEX-API

Regards, Steve.