Semantic HTML means using HTML elements that describe what a part of a web page is for. For example, a page can use elements for its main content, navigation, and individual articles instead of using the same generic container for everything.
It is like labeling the rooms in a building, so people and tools can tell where to go. Meaningful structure helps browsers, assistive technology, and search engines understand the page. It does not automatically make a page accessible or easy to use, but it gives those tools a clearer starting point.