The idea that insiders have information not available to the market. Moves made by insiders can signal information to outsiders and change the stock price.

The thinking goes that if a high level executive, such as a CEO, is selling, he or she is probably doing so for a reason that you, as the public, don't know yet, so you should get out also. The same is true for the opposite. If an insider is buying stocks in his or her company, it signals outsiders to also buy based on the idea that the insider knows more than he or she is letting on to the public.