By "form" Dair means the distinction between CAPITAL letters and its lower case equivalent, or a regular letter and its italic variant.
Contrasting uppercase against lowercase depicts formality against casual. Uppercase letters start sentences or are often used as Acronyms. Too many capital letters and there is loss in legibility and makes people think you are yelling at them. Contrasting with Italics also, helps draw attention or create emphasis to a certain word or words. Working with form allows some freedom with design, as layout doesn't need to be changed to draw attention to a piece of text of the same size.
Take a look at the quotation below:
In reviewing the quotation above, the eye reads through, but takes not of the word "WORKS". As a visual cue, the letterform contrast awakens the importance of a specific word or words. The eye also takes note of the "Looks" and "Feels" like but you automatically know that they don't mean as much as the "WORKS" word.
Now take a look at the same quotation but in italics instead of uppercase:
Notice how it gives the same emphasis as using the uppercase letters, but using italics instead. You still get the same idea as the first one, just in a different form.