Everything before the first <h1> element is considered to be the abstract of your presentation. It is rendered with a heading depending on the language (i.e. 'Summary' in English) and is enclosed in a <div class="abstract"> block. You can change it's appearance in the stylesheet.
Every slide should have a title and, optionally, a subtitle. You define them with a <h1> element, resp. <h2>. The input file (master document) is split into slides at every occurence of an <h1> element. This means, for the headings on your slide you have to use the <h3> element.
Let's see an example:
<h1>Master document structure</h1>
<h2>My first slide, or: «Look, Ma, no hands!»</h2>
<h3>Please note:</h3>
<p>Although the subtitle can appear anywhere between two <h1> elements, you should place it immediately after the preceding <h1> element, just to make things more readable.</p>
<h2>My first slide, or: «Look, Ma, no hands!»</h2>
<h3>Please note:</h3>
<p>Although the subtitle can appear anywhere between two <h1> elements, you should place it immediately after the preceding <h1> element, just to make things more readable.</p>
This will produce the following slide...