Good headings express the general idea of the content below them, and search engines use headings to get an idea of what your post is about. In Blogger, you can now designate text as "header" text, and the search engine will know that the text you’ve selected represents a summary of what you’ve written.
There are a few header tags you can add to your text, and all they do is define the hierarchy of the headers and accordingly change the font size. For example, here is a list of headers (in order of largest to smallest).
<h2>This is my first heading, about the general topic</h2>
<h3>This is a subheading</h3>
<h4>This is a subheading underneath a subheading</h4>
To designate text as a header, just highlight the text in the Post Editor, and click on either “Heading” (<h2>), “Subheading” (<h3>), or “Minor heading” (<h4>) in the toolbar. If you like, you can verify the HTML you’ve added by clicking on the HTML view.
For more background on SEO, read through this article in the Webmasters Help Center, and be sure check out their SEO Starter Guide (pdf).