The Real Importance of SEO

I have been going from one forum site to another for quite some time now and as far as I can tell, there is always a topic about this thing that they call as SEO. I have signed for this forum site in order to get to know about this SEO or search engine optimization thing. What would really make this a source of attention for the majority of the online community? Is it really that important? If yes, can you list down the things that really make it a vital part of the lives of webmasters and the existence of those websites we see on a daily basis?