The <hr> tag in HTML stands for horizontal rule and is used to insert a horizontal rule or a thematic break in an HTML page to divide or separate document sections. It represents a thematic break between paragraph-level elements, such as a change of scene in a story or a shift of topic within a section. The <hr> tag is an empty tag, and it does not require an end tag. It is a block element, which means it starts a new line and uses the full width of the page. The <hr> tag supports additional attributes like align, noshade, size, and width. The width attribute is used for specifying a width for the <hr> tag, and it takes pixels or percentage as a value. The color attribute is used to specify a color for the <hr> tag. The size attribute is used to define a height for the <hr> tag. The align attribute is used to specify the alignment of the horizontal rule. The <hr> tag is supported by all major browsers.