Meta tags describe the content of a web page, but they don't appear on browsers. They are intended to let search engines know that the content is about, and users can only view them in the source view. An example of meta tags displayed in source view is like below.
A search engine like Google crawls the web page, index the page, and display the results in SERP like below:
The meta tags are placed in the <head> tags within HTML, and the elements begins with meta, i.e. <meta description="...">. SEO is all about contents, and meta tags help search engines what to look for in your web page. The notable meta tags that help SEO include:
There are many more meta tags that you can add to your HTML to describe contents of your web page. Additional meta tags include author, open graph, content-type, and many more.