A Force scream was an involuntary Force exertion manifested by those whose alignment was with the dark side of the Force. It was a scream, of strong frustration, rage, or grief, emitted through the Force.
If a Dark Jedi or Sith Lord was being beaten and frustrated by his foe, he might let out an enraged scream (amplified through the Force) that triggered shock waves that rippled through the Force. This caused devastation to any that were unfortunate enough to be in proximity. Like many dark side Force powers, it allowed the user to use powerful negative emotions as a weapon, albeit involuntarily in this case.
The Force scream had been called «a wave of hatred, amplified and fueled by the dark side, that is capable of smashing through mental and physical defenses with ease.» A number of references to Dark Jedi in Old Republic archives often mentioned the Dark Jedi violently losing control in the final moments of battle.