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What it means is that it should have the right keywords, have relevant and clear content and a well written source code. Essentially, by having these the search engine can easily read your site. You should also know that aside from the keyword requirements, search engines like Google always change their rules in determining the rank of a web page. The reason they do this is tom make sure that users will find exactly what they are looking for. In other words, the quality of the result of the searches is the major consideration of the search engines.