What does the nth-child selector do in CSS?

The nth-child selector in CSS is designed to select elements based on their position within a parent element's list of child elements. It allows for targeted styling of elements that match a specific pattern according to their order number. This can be useful for applying styles like background colors, margins, or other properties to every nth child element, which can help create visually appealing layouts or effects without the need for additional classes or IDs.

For instance, if you wanted to style every third list item in a series of list items, you could use the nth-child selector to achieve that with a rule like li:nth-child(3n).

This functionality provides significant flexibility in design, enabling developers to fine-tune the appearance of elements based solely on their sequence in the document structure, without altering HTML markup.

The other choices do not accurately describe the function of the nth-child selector, emphasizing its specific focus on element positioning rather than attributes like ID or class.