What are the meta tags?
Meta tags are informations located in the part "head" of a HTML document.
The content of these tags is not directly displayed on the pages visited, which makes this information rather invisible to users.
They are visited by browsers, some softwares and obviously, by search engines.
There are a lot of meta tags but we have to admit that the great majority of them no longer play an important role in the ranking of a website.
It is worth pointing out that for the "purists", we should rather talk about "meta element" than "meta tag" in order to strictly meet with the specifications of the W3C.