For the person who doesn't know too much about SEO or Internet Marketing in general, it can be hard for it to be explained to them. This is especially difficult for any marketer to explain to the potential customer when you're trying to sollicit your service. Unfortunately, if you don't know how to break it down to them, they certainly won't feel comfortable enough to take up on the offer. It's just one of those things, and you can't blame them, no one buys things that they don't really understand. And with marketing services not being cheap, it's imperative that you explain to them how the process works — using things that they understand.
Most people, such as SEO companies, like to explain it by saying that SEO is the way in which search engines find a specific website. This would all be done with all of your on-page work, such as tags and descriptions. And when it comes to off-page SEO, such as link building, they may liken it to a conversation that many people are having about a certain topic. The more people that are talking about it, the more relevant it is to the conversation, giving it credibility. So, to break it down, the search engine needs to discover it. While that may be a muddled mess, a lot of people have had success in explaining it to the person in need of a web presence. There are many ways to explain it, but this is just a starting point.
While this explanation may not convert may people, it definitely gives the potential customer thinking in the right direction.