META tags are optional lines of HTML code in the HEAD
section of your document that can be used to describe your
page. META tags provide descriptive information about your
site to search engines, but in a form that isn't displayed
by the browser. 

The relevant attributes for search engines are the
DESCRIPTION and KEYWORDS attributes. Only 25% of web sites
include META tags, and not all search engines use them to
rank pages, but many do. 

Meta  means "Information about information"

for every webpage it is  must because the search engine can 
search the relative webpages based on metacontent of the 
document.
in meta tag they used to mention some importent  keywords
in the web page.