Congressmembers have access to the information earlier than the public. For example, Senator A may be reading a private draft of Bill B, which he expects will benefit the stock price of Company C, so he buys shares of C now at $50, unveils the legislation in a week which pushes the price up to $75, and passes the legislation the next week pushing the price up to $100. So the insider-trading Congressman gets a $50/share profit, while someone who was diligently watching the hill would only be able to act from the public release of the legislation and capture a $25/share profit.