SEO stands for search engine optimization, and while there are many detailed explanations, the best one we can think of is “making things appear higher in Google (or any other search engine)”. In our case, we also break SEO into on-site and off-site SEO.
On-site SEO works on things on your own site that will help search engines understand what you do and find all the pages on your site. Off-site SEO is concerned with creating links back to your site in order to help demonstrate relevance and popularity to Google.
It is obviously more technical than that, but these are good summaries.