Description is the name of a tag which can be located in the head of html documents.
It let’s the search engines know, what the content of the page’s about.
It’s one of the most important factors in SEO case.
Keyword is the name of another tag with the same usage.
The only difference between description and keyword tags is that in the description, you describe the meaning of the page in a regular sentence format.
But in case of keyword, you sort number of keywords which you’d like people to be referred to your page while searching them on the search engines.