You need as many people as possible to see your website. You would also like for your website to show up when people search for keywords associated with your website. For example, let's say that you have a farming website. People can do a Google search for "farming". But "farming" is such a general word that a Google search will give you over 49 million results. WOW! That's a lot of results. Your website is somewhere in there (maybe the 43rd millionth result). So what makes your website get to the top?
There are a number of things that can be done. But remember that websites don't just magically end up first in search engines. It takes time and also a lot of keyword research. Now imagine that we place some well researched meta data on your website with words in addition to just "farming". So maybe it might be "St. Lucian farming" or "Caribbean farming". Now the Google search boils down from 49 million to 13 million results. Not bad eh?
Of course, there are other things that would be done as well. In addition to submitting websites to various search engines like Google and Bing, it can be submitted to places like Alexa which is a website that monitors ranking and traffic.
If you would like more information on how SEO works, visit the MOZ website. They are awesome specialists on the topic.