Gorgons were seriously ugly ladies from Greek mythology, best known for having snakes for hair and being able to turn anyone who looked into their eyes to stone. You can read more about them here. The most famous Gorgon was Medusa, who was killed by a hero named Perseus.
At several points in Libation Bearers (835, 1048a), characters refer to Clytemnestra as a Gorgon. Also, at lines 831-832, the Chorus tells Orestes that "[You must be brave], and in your heart / keep up the courage of Perseus." What these references are getting at is clear: Clytemnestra is one mean lady.
It is also possible that, by bringing to mind the Gorgons' power to turn people who look at them to stone, the Chorus is warning Orestes not to be frozen into inaction when he confronts his mother. Of course, Clytemnestra doesn't really have magical powers, but she does try to use persuasion and emotional appeals to freeze her avenging son in his tracks.