What does Macbeth see at the banquet that frightens him beyond measure?

The banquet scene to which you refer occurs in act 3, scene 4. It immediately follows the scene in which the murderers sent by Macbeth have succeeded in killing Banquo but have been unable to kill his son Fleance also. When the banquet scene opens, then, the audience already knows that Banquo is dead. At the beginning of the banquet scene, the murderers relate what has happened to Macbeth, who says he feels beset by “saucy doubts and fears.”
When they leave, the ghost of Banquo appears and sits in Macbeth’s seat at the table. Nobody else can see the apparition; at first Macbeth seems to think somebody has arranged this to frighten him. He then addresses the ghost: “never shake / Thy gory locks at me!”
The noblemen assume Macbeth is not well, but Lady Macbeth tells him the ghost is merely “the very painting of your fear” and to disregard it. Macbeth, however, continues to maintain that he has seen the ghost and that Banquo and other murder victims may return “to push us from our stools,” or usurp power.