page in a web search engine's unpaid results—often referred to as "natural", "organic", or "earned" results. In general, the earlier (or higher ranked on the search results page), and more frequently a website appears in the search results list, the more visitors it will receive from the search engine's users; these visitors can then be converted into customers. [ref. Wikipedia}
To break that down to layman's terms... you want your business web site to show up on the top of the search above your competitors. Say Sue and her 8 month old baby are in Racine, WI and her car breaks down in the middle of a busy intersection. Sue puts in a search term for a tow service, gets her results and makes the call.
The web site Racine Towing Services must be optimized for the term "tow truck racine wi" as Google has this site as the most relevant site for the search. Maybe the site might be relevant for some other search terms as well, like towing racine a couple other terms related to the business. So if this business gets the call first, they are most likely going to be able to turn this call for assistance into a call for service.
Now that was just an example, and even though it looks simple, it is quite complex. We will cover some more in depth posts on what to look for in your site to help with its relevance, or in other terms, (rankings) to your site on another post.