The :first-child selector matches elements that are the first child of their parent. This is a powerful CSS pseudo-class that can be used with querySelectorAll() to find specific positional elements in the document. For example, document.querySelectorAll('li:first-child') would select all list items that are the first child of their parent. This is different from :first-of-type, which selects elements that are the first of their specific type within the parent.