A so-called truism of business, it refers to gaining a powerful competitive edge by being the first company to establish itself in a market (or by being an early successful player). In other words, a company attempts to become the most popular brand name within an industry segment due to the fact that it was the first to get its brand name known as the brand to buy within a particular niche. Referred to by some execs as a "sprint-and-spend strategy," it requires a large amount of marketing dollars to launch a new brand. Quite simply, it means getting a head start on everyone else in your industry.
An example is Amazon.com, which grew to be a national brand in just a few short years. It seized first-mover advantage by getting its products and brand name first-to-market within the online book industry, thus grabbing a large percentage of market share (despite the fact that it was not the first or only bookseller on the Web).