SEO stand for Search Engine Optimization. It's a fancy term that means a website is user friendly to search engines bots. When you use SEO on a site you fix the site's meta tags, title, discription, and even the body of the index page, among other things to give the bots the best possible idea what your page is about and how relevant the content is to everything else on the page.
Most search engines have a submission form making it easy to get you listed from a few days to a week and often a site with SEO techniques on it won't matter at all for them. You won't be at the top of the rankings, but that's the price you'll pay for doing it for free and with a 2nd tier or lower search engine. Even Yahoo has submission forms, and now multiple of them depending on the purpose of the site. Google however, has surpassed Yahoo in usage, making it the current #1 search engine in the world, so it's really them you want to be listed in.
Google's search engine bot is automated. There are no more manual submissions put into the Google index anymore. It's been said using their submission form could take a week to years to getting listed to not even at all. It's bot crawls the internet looking for websites to index according to their relevancy. As it combs through your pages it looks for words that tell it how your site should be indexed and if the words it finds really are what your page is about. InBound and OutBound links need to be relevant to your page as well. If your site is not relevant, or not relevant enough, it may move on and not list you at all. So, basically, Google is at the top of the mountain and you want your site Google bot friendly.
On my first site, I thought I had done this correctly, but to this day it's not listed in Google. After having some inspiration for anotehr website I openned a new page. I was picky with sites linking back to me and careful keeping each page slightly different in subject matter, but just enough to hit other keywords the Google bot may like when seeing if it was relevant. It was already preset with SEO as a habit so when the bot came along, everything it found through the SEO, it would find onsite. Sure enough, I was listed very, very quickly. Especially when you compare it to the zero listings of my first site in a much longer period of time.
It's really amazing how quick the bots add you when it's done correctly. The extra time it takes to make sure everything is just right more than makes up for the extra time it'll take to get it listed and indexed the hard way.