SEO stands for Search Engine Optimization and you may have heard about it. Simply put, it is the activity of attempting to improve the ranking or visibility of a website organically in Search Engine results. (Organic traffic is any traffic that you do not have to pay for)
Basically a search engine, like Google, uses a crawler (a program that crawls the internet searching for content) to gather all of the information about all content it finds on the web and passes it into an algorithm which then tries to match that content to the keyword search that a user typed into Google. It is a complicated process which takes many factors into consideration before your page is ranked and displayed to the end user.
Now that is the SE(search engine) portion of the equation. The 'O' in SEO (meaning Optimization) is where the programmers and creators come into play. In order to show up higher in Google rankings, we as programmers, SEO guru's or whomever else is involved, need to make your content more relevant and easier for search engines to understand. Building sitemaps, creating the perfect Meta tags, and optimizing your entire website for search engines will help your website get noticed. Even structuring your URL's for easier readability has a minor impact on your page's ability to rank. Though only a small fraction of the total SEO equation.
From title tags and proper URL formatting, to creating a sitemap and submitting it to Google or following SEO best practices, there are many factors that come into play when trying to rank a website on search engines like GOOGLE or BING.
In the end SEO will help bring your website to the top of the ranks!
Also Submitting a sitemap to a search engine like GOOGLE is much faster, instead of waiting for their crawlers to find them on your website. Which means your website will rise in ranking much faster with a submitted sitemap.