Search Engine Optimization (SEO) is the art and science of writing and programming a web site so that search engines can find that website most easily. This is also known as Search Engine Marketing (SEM). When someone types a phrase into a search engine, the search engine is programmed to seek out that phrase on other websites. The key words must appear enough for the search engine to notice it, but not so much it gets filtered out spam.
The search engine filters out odd or strange word combinations on a website to differentiate, for example, a shoe store from a restaurant. Since web designers can just fill a page with popular search terms to increase the traffic to their site, the search engine is programmed to ignore those tricks. This is good for everyone, as you only want customers who are interested in you anyway.