When your web site is found by a search engine like Google, they look for certain pieces of information to display in their results. HTML meta tags are one of the first things they check for.
If you don't have the meta tags in place the search engine will just display an excerpt of text from the page instead. It might be relevant and interesting to the person searching but usually it's not.
Basically, if you don't use the HTML meta tags then you have no control of how your site listing appears.