The <HR> tag produces a horizontal line the width of the browser window. A horizontal rule is useful to separate major sections of your document.
You can vary a rule's size (thickness) and width (the percentage of the window covered by the rule). Experiment with the settings until you are satisfied with the presentation. For example:
<HR SIZE=4 WIDTH="50%">displays as:
By the way, why didn't the browser interpret the <HR> tag that appeared in the first paragraph and produces a horizontal line? In other words -- why didn't the page look like this one?