Can anybody tell me that why we use meta tag in html
Meta tags are used for many things. The most important use for meta tags would be probably for SEO purposes. The meta description is probably one of the most important parts of the meta tag because that is what shows up under the title on Search Engines such as Google or Yahoo. The following link will give you more information about SEO and how you can use the meta tag. https://blog.kissmetrics.com/website-source-code-seo/
The meta tag can also do many other things such as set the author, viewport and keywords of a web page. Here is a link that will tell you a lot more about what the meta tag is used for. https://www.w3schools.com/Tags/tag_meta.asp