Semantic Tags
In programming, semantic refers to the meaning of a piece of code. For example in HTML, each of all semantic elements like form, article and nav tags has a specific meaning so that it can indicate what purpose or role that element has.
You could write a code like this and the browser will render it to look exactly like a top level heading,
<span style="font-size: 32px; margin: 21px 0;">
Is this a top level heading?
</span>
but it's not a good idea to use these codes instead of h1 tags because they don't have any semantic values. HTML tags should be coded to represent the data it contains. By using appropriate HTML semantic tags, you can write a much cleaner code with easy readability. Also, it could give your website a great accessibility because the Search Engines will be able to understand the keywords of your website and its content, and recommend them to thousands of users.
Semantic Web
According to 2001 Scientific American article "The Semantic Web" (Berners-Lee et al.), the definition of the Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation. The goal of the Semantic Web is to make Internet data machine-readable so that they can easily be processed by machines. With Semantic Web, data could be quickly interpreted by machines and allow them to carry out numerous tedious tasks like discovering, blending, and taking action on the information available on the Web.
Reference
developer.mozilla.org/en-US/docs/Glossary/S..
w3.org/RDF/Metalog/docs/sw-easy
en.wikipedia.org/wiki/Semantic_Web
techopedia.com/definition/27961/semantic-web