The pediment is the triangular place under the roof of a Greek temple. Each temple has two pediments, one on the front and one on the back. They’re always isosceles triangles. At first pediments were probably plain, but soon the Greeks began to decorate pediments with stone sculpture. One of the earliest pediments, on the temple of Artemis on the island of Corfu, has a scary Medusa on it.
Later Greek sculptors put whole scenes on pediments, usually one exciting part of a well-known myth. The Parthenon has the story of the birth of Athena on one side, and the story of Athena and Poseidon fighting to be the main god of Athens on the other side.
The tricky thing about pediments was what to put in the little angles of the triangle at the sides. On the pediment of the temple at Corfu, the sculptors have Medusa standing, then animals who are lower, and then teeny little people in the angles. On the Archaic temple at Aegina, the sculptors had fallen soldiers lie down in the angles.