A paragraph in HTML is a container holding contextual content, the written word. The HTML of a paragraph opens and closes with the paragraph tag.
<p>This is a sample paragraph.</p>
If we were writing exclusively in HTML, every paragraph would have to have that tag wrapped around it.
In the WordPress Theme, specifically the stylesheet, the paragraph is styled to feature whitespace between the words and the paragraph above and below as well as within the paragraph itself, adding spacing between the paragraph and what is in it such as images.