If you do not put a description tag, what appears in the listing would be likely to be whatever first appears on your page, usually the alt text of some graphic or banner or perhaps your top menu.There are search engines which also look for a META Robots tag to determine how they index your site.
In a website page Title and description is very important because this are the information that google pull to display on SERP (not all the time though because some do not put meta description on their page they let google to the indexing). Title, description, content and its keyword should be all well associated with one another.
Meta tags are HTML codes that are inserted into the header on a web page, after the title tag. In the context of search engine optimization, when people refer to meta tags, they are usually referring to the meta description tag and the meta keywords tag.
I use Wordpress for my sites. When I write a new post or edit a post, I also write meta tags. It is very easy to find where I can write meta tags.
Tags are keywords in SEO.
if you do not have a META Description tag for your page, some search engines will simply grab the first few lines of your page to serve as the description of your site. This description is more important than you may think.
If you view the HTML sources for the web pages on the website, you would have noticed that they all contain META tags. META tags influence the way some search engines list and index web pages.
There are at least three META tags that you will want to use to make sure your page is search engine ready.