I am absolutely unfamiliar with /robots.txt what is it? what should i put and how it is important for search engines?

It's not all that important to search engines, they will spider your site w/o one, though if it's there they will use it - or should. There is a ton of information on the internet about robots.txt, do a search for it. The file should go into the root directory of your site. It can be used to STOP spam bots and spiders from indexing all or part of your site. Unfortunately, there are many bots that will ignore the robots.txt file too.

Here are a few sources:
http://www.javascriptkit.com/howto/robots.shtml
http://www.robotcop.org/
http://www.robotstxt.org/wc/robots.html
http://www.robotstxt.org/wc/active/html/index.html
http://www.psychedelix.com/agents/index.shtml

You only need one if you want to stop bots from accessing certain parts of your site.

www.robotstxt.org

Generally speaking, whenever a spider visits your page, it follows all the links in your page. so all of your pages linked in a logical manner then all the pages in your site will be indexed by that particular search engines. If you want to allow spiders to index particular pages and not to index some other area of your site then you have to use robots.txt.
For example, you may not want to allow spiders to index your images, scripts,etc for that you have to disallow spider using robots.tx