You can go to jail if convicted by a criminal court of insider trading.
It is very difficult to prove "beyond reasonable doubt" which is the test for criminal case. In a civil case such as Fyffes vs. DCC, the test is "on the balance of probabilities", I think.
Trading in the shares of a public company whilst having insider information. Insider information is information which is not yet in the public domain ie not published but which would affect the price of a share if it were publicly known. For example if sales are going down but this has not yet been notified to the public, then it is likely that the share price will go down when this is known.