In the past few years, there have been major changes in the hidden code used to create web pages. This isn’t a change that you would notice by looking at a website, so it doesn’t seem to matter for most people. But it really does matter for the future of the web and the future of your website. The changes are in adding semantics to the code used to build the pages.
Without getting too technical, it’s a way to describe what is on the page in a way that computers can understand.
Who cares? You do, if you want search engines to understand what is on your site. And you want them to understand your site if you care about ranking high in search results, presenting your search result in a nicely formatted way so it is appealing for a searcher to click on when your site comes up in the search results.