It is a piece of code you can add to a page’s HTML to help search engines understand what is your website about & what type of information it contains. It pairs up words with specific values that help the search engines to categorise & index your content. Basically, instead of hoping Google understands what you are, you tell it directly through pieces of code that the search engine robots are able to read, process & use to return more informative results to users.
For example, if you’re a dentist or brick & mortar store, you might want to point that out using the proper tags so you won’t have to rely solely on keywords.