To italicize text is to put it in an italic typeface. In APA style, titles of books and names of journals are italicized, but titles of chapters and articles are not.
This is a normal typeface:
Aa Bb Dd Cc Dd Ee Fe Gg ...
This is a italic typeface:
Aa Bb Dd Cc Dd Ee Fe Gg ...
Tip: To italicize text in Microsoft Word, select it and press Ctrl + I (or click on the I button on the Home ribbon).