Based on the suffix .com, this refers to a company in the industry (with a .com Web site) whose primary focus is on the financial aspects of taking the company public (versus addressing any real market need or establishing a successful, long-term business model). Activities such as IPOs and grabbing market share (by getting first-to-market) became mantras for many dot-com execs, ultimately leading to their downfall (see: new economy). While hundreds of dot-com companies are succeeding, it seems that since the dot-com shakeout, the phrase "dot-com" is used more often for the firms that ended up in the dot-com deadpool.
An example of its usage goes like this: "He met with plenty of young American dot-com executives overseas, and they simply weren't prepared to do international business. They were just hemming and hawing, trying to convince everyone else they'd succeed."