Italicize simply means writing something in italics.
Italics itself is a writing style whereby the letters lean to the right.
Underline is refers to drawing a line under a statement or a word.
The aim of using italicizing or underlining a statement or word is to emphasize it, show its importance or direct attention to it to lay emphasis.
The title of a larger piece of work is usually italicized or underlined so as to emphasize the level of its importance and how interesting it is so that readers will not choose to miss it.
