The HTML Specification or Standard is sort of a rule book that's agreed upon (not all the time) about how browser vendors should implement HTML Tags and Elements.

The Standard is quite monstrous, so I will not encourage you to go read the whole thing (for now), find anything that you are curious about? That's an example of when you should check the standard, study the section that's of interest to you and then leave. That's it.

The Spec (or Standard) usually contains the algorithmic description of tags and also where they can be used, when they can be omitted and sometimes which part of the tag can be omitted in a particular situation when a certain condition is met.

The Specification also states where to place a tag in a document, the context in which the tag can be used, what particular element can follow it, its attributes and some advice to web developers (you) on some attributes you can give an element to aid accessibility. An example of this is:

"Authors are encouraged to specify a lang attribute on the root html element, giving the document's language. This aids speech synthesis tools to determine what pronunciations to use, translation tools to determine what rules to use, and so forth" - HTML spec.

This is just making it clear that the html element should specify the language that the document is written in to make it easy for screen readers to read your document and even which pronunciation to use.

Some other information contained in the specification are micro data, how web pages are loaded, the html syntax and even how elements are rendered.

The specification can be accessed online.

Edited September 19, 2020: Grammar fix.