Meta data, often called meta tags, are data about data. They are used to briefly summarize the contents of a document which gives search engines an idea what a web page is about. Further on search engines decide if this page comes up among search results for a specific keyword. Among the most popular and widely used kinds of meta tags are meta descriptions, meta keywords, and met robots.
The meta description is the kind of meta tags SEO usually requires and one of the most important meta-elements. If the text in a meta description matches the request made by a user in a search system, search engines display this text among the search engine results page (SERP). When you create a meta description, you need to keep in mind that on average the text shouldn’t contain more than 155 characters. A longer description won’t make search engines kick your page out of the SERP, but a part of the text will be cut off.
An appealing, well-written meta description can have a big influence on the click-through rate of the users. Since each URL should deal with a separate topic, each URL should have its own description. You can find tips on creating the Meta-Description in Google Help.
Meta keywords should originally be used to tag a text. For this purpose, particularly important terms were placed within the meta-keywords specification. In good old times at the very beginning of the Internet history meta keywords had quite a weight in the calculation of page ranking, but today they are not that important. However since September 2012 a new meta tag called “news_keywords” was launched by Google.
Meta robots are powerful elements, e.g. they can be used to prevent a single URL from appearing in the search. In this case the "Noindex" meta tag is used for your SEO campaign. Using the “robots” meta tag you tell the robots if they are allowed to index the given page and if they can use the links present at the said page. You need to use this tag on every page you want to cut access to.
Are they really important for SEO?
Yes and no. In theory they significantly increase chances that your page will go up in SERPs. But this only happens if meta tags are used wisely – a general rule that applies to everything. Search engine robots need to have a simple idea of what your page is about which meta tags can provide. However it has to be then proved that the meta tags you use are related to the real content of the page.
There’s no actual limit to the number of meta tags you use, but you should be aware that virtually ‘littering’ the page with them doesn’t help anymore. That’s when you involuntarily pay for other people’s mistakes: after web marketers abused the use of meta tags some time ago search engines got more suspicious of over the top use of this tool.