The “ocean” refers to the market or industry. “Blue oceans” are untapped and uncontested markets, which provide little or no competition for anyone who would dive in, since the markets are not crowded. A “red ocean”, on the other hand, refers to a saturated market where there is fierce competition, already crowded with people (companies) providing the same type of services or producing the same kind of goods.
Their idea is to do something different from everyone else, producing something that no one has yet seen, thereby creating a “blue ocean”. An essential concept is that the innovation (in product, service, or delivery) must raise and create value for the market, while simultaneously reducing or eliminating features or services that are less valued by the current or future market.
More info: http://www.blueoceanstrategy.com/downloads/bos_web.pdf