Meta Tag is an HTML tag of a web page which gives information to the search engine about the metadata of the web page such as title, keyword and description so that the search engine understands what this web page is about. Search engines decide the rank of a web page in ranking on the basis of meta data and quality content.
For example, search engines get information about the author of a web page through tag and web browsers get information like viewport through tag which helps them to present the web page correctly.
The following information can be provided through tags.
Description - You can provide description of the web page through Meta tag. This description is shown in search results by search engines. When your web page is shown in search results, this description is shown below the page title. If you do not define description through Meta tag then search engines use the first few lines of the article as description.
Keywords - You can provide information about keywords related to your web page to search engines through Meta tag. Keywords are important for ranking in search results. Keywords let search engines know about which topic related information has been provided on a particular page.
Author - Information about the author of the web page is also provided by Meta tag.
Last Modified - You can also provide this information about when the web page was last modified through Meta tag.
Character Set - You can also provide information about the character encoding used in the document through Meta tag.
Meta tag is also very important from the point of view of Search Engine Optimization (SEO). Also, through tag you can control the presentation of your web page on screens of different sizes.