HTML tags come in matched pairs that act as containers for a section of text. (In older versions of HTML, not all tags come in pairs.) If tag like <something> has a matching tag, the matching tag will look like </something>.
For example, a paragraph of text is "bracketed" by the tags <p> and </p> .
Most browsers are very forgiving and let you get away with grammatically poor HTML. For example, you can usually omit the ending tag </p> of the paragraph pair. But is is a poor idea to do so, because as HTML becomes more complicated (as it will) browsers and other applications will require following the rules.
Is the following text one paragraph or two?
<p> I had called upon my friend Sherlock Holmes upon the second morning after Christmas, with the intention of wishing him the compliments of the season. He was lounging upon the sofa in a purple dressing-gown, a pipe-rack within his reach upon the right, and a pile of crumpled morning papers, evidently newly studied, near at hand. </p>