Web standards are a set of specifications about how to write HTML code for websites in a standarised way. This means, which tags to use in each part of the website, like paragraphs, lists, titles, quotes, layout, and so on, so they make sense, and give context and structure to the content.
With no styles, a website can be seen in its bare structure, the way other computers (such as search engines and screen readers) would see it. It also means that you can see the most important part of the website, the content, in its primary state.
For us developers, seeing a website in its bones help us make better decisions about how to write our code to achieve good accessibility, and good SEO performance.