Glad you asked there are probably others wondering the same thing.
H2 and H3 are titles, H1 is another one, used in WordPress. These titles usually have different size and sometimes different font.
So you will write you main text in “paragraph” when it comes time for a new part of the article you can use the different titles to break things up.
If we look at the above article the text “Broad Keyword” is using a H2 title. The text “What Do People Click on in the Google Results?” is using a H1 title. The text “I’ll take a first page ranking…” is using a H3 title.
As you can see all these have different size text.

Hope that cleared things up for you,

Vicky

Cheers!

Great question! I try to include the full keyword phrase in the title, first and last paragraph. It doesn’t always work out with the title, want to make sure you have something that people will want to click. If it fits naturally in a H2 or H3 header I might use it there as well. Through out the article, parts of the keyword will be used naturally if the article is kept targeted.

Hope that helps,
Vicky