SEO is getting your web site optimized in a way to rank high in search engines, there by they will reward you by basically sending traffic. SEO is not really simple, but the bottom line is, if your page is optimized both on page and off, you generally get higher natural search engine rankings, which leads to traffic for you.
Off Page SEO refers to the external factors which affect the ranking of your website such as the analysis of keywords and external links pages. Off-Page SEO is difficult to establish and maintain but is critical in order to achieve top rankings on all of the popular search engines.
On Page SEO refers to the controllable internal factors which affect the ranking of your website such as the coding and accessibility of your website. For example, the correct use of titles, headings and copywriting.