Once you have divisions, what do you put into them? Most likely, paragraphs. We use long, exact names for tags that are used infrequently, like <subsubsection>. But for frequently used elements, we use abbreviated tags, often identical to names used in HTML. So a paragraph is delimited by simply the <p> tag.
Lots of things can happen in paragraphs, some things can only happen in a paragraph, and some things are banned in paragraphs. Inside a paragraph, you can emphasize some text (<em>), you can quote some text (<q>), you can mark a phrase as being from another language (<foreign>), and much more. You can use special characters like an ampersand (&, <ampersand/>) or an octothorpe, aka “hash tag” (#, <hash/>). You must put a list inside a paragraph, and all mathematics (Section 3.5) will occur inside a paragraph. You cannot put a <table> or a <figure> in a paragraph, and many other structured components are prohibited in paragraphs.
Paragraphs are also used as part of the structure of other parts of your document. For example, a <remark> could be composed of several <p>. As you get started with PreTeXt, remember that much of your actual writing will occur inside of a <p> and you will have a collection of tags you can use there to express your meaning to your readers.