Meta tags are pieces of information that are hidden behind a webpage. A user viewing the page won't see them on the page, but search engines will read them. So they are a way for you to provide search engines with information about your page.
The information in these tags are also shown in search results - so how you write them can impact whether or not a person will click through to your page.
The title element of a web page is meant to be an accurate and concise description of a page's content. The contents of this tag are generally shown as the title in search results. This is also shown at the top of the browser window.
Google typically displays the first 50-60 characters of a title tag. Keep your titles under 50 characters.
Meta descriptions are meant to provide concise explanations of the contents of web pages. Meta descriptions are commonly used on search engine result pages to display preview snippets for a given page.
Meta descriptions can be any length, but search engines generally truncate snippets longer than 160 characters. It is best to keep meta descriptions between 150 and 160 characters.
This tag is no longer used by Google. Other search engines use it but apply very little weight to it. This tag can be used by search engines to identify websites that are SPAM - and to remove them from search listings. So only use this tag if you understand the implications. If you are unsure, leave this tag blank.