SEO stands for "search engine optimization." The world of SEO helps websites build rapport with Google's ever-changing search algorithm: http://bit.ly/1al5P4a. Obviously, there are other search engines out on the Internet, but Google is the focal point for SEO specialists.
Think about it: how often do you venture to the second page of Google's generated search results to find what you're looking for? If honest, rarely. So, as far as digital marketing is concerned, the battle is for position atop the first page of Google's search results.
If that can be accomplished through "white hat" SEO techniques, increased corporate revenue is bound to be a direct byproduct. Anyway, here's a helpful infographic which might help things make a bit more sense, as well: