SEO stands for Search Engine Optimisation, the art of optimising the factors used by search engines to determine where your site, and the individual pages on your site, will rank for search phrases.
SEO can be broadly broken into two main areas, onpage factors and offpage factors. Obviously onpage factors are easier to affect than offpage, as you have control of what is on your site. Offpage factors can be affected to a limited extent as well, it just requires more work.
How well you will rank will depend on many things, including your content, your site design, the age of your site, and whether the links to your site are quality or not.
Which industry or sector you are in is also important. For example, if you put up a site that sells "Blue Widgets made in Timbuktu", and there are only twenty other sites that sell those, then after your site has "aged", you can expect to rank fairly well when people search for those Blue Widgets made in Timbuktu. However, there may be twenty million sites selling "Blue Widgets", so it's unlikely a new site will rank anywhere near the top when people search for generic Blue Widgets.
There are many factors involved in SEO, and the difficulty is discovering which ones are important and how they are weighted. The SEO community is in many ways in conflict with the search engines. The search engines don't want website designers to know how they rank sites, because people who create spam sites would immediately use that information to rank their sites at the top of the SERPS (search engine result pages).
Because people do eventually discover, broadly, how search engines rank sites, search engines constantly evolve the algorithm they use to rank sites. SEO changes to keep up, what was cutting edge six months ago may be obsolete, or even penalised, today.
The only consistent key to good SEO is good design and good content.
There are many myths about SEO and how to instantly rank your site in Google and other major search engines. Unfortunately there are also those who try to take advantage of site owners by making claims about getting their site ranked well. These should really be regarded as like "get rich quick" schemes.
Some myths are based on information that was correct at one point, or that was partly correct. There is no magic formula, but there are ways to increase your site's chances of being ranked.