A libation is a liquid offering, poured out to a god or goddess. They would usually be an offering of wine, water, milk, or honey. In this scene, it is gods themselves making a libation.
There are three deities shown in this image. In the middle is Zeus, king of the gods. The throne and long beard are typical in representations of him and the thunderbolt in his hand confirms that it is him. Zeus ruled over the earth and other gods; he is the father of many gods and heroes. In front of Zeus is Athena, one of Zeus' daughters. She is recognisable from the combination of a female form and clothing with a helmet, spear, and her special cloak, known as an aegis. Athena was patron of many cities, including Athens. She presided over matters involving wisdom, knowledge and technical skill, especially crafts and war. To the left is a winged goddess. It is hard to be certain who this is as there is more than one winged goddess in ancient Greek culture, but it is more than likely Nike, the winged goddess of victory. She brings success to those competing in all sorts of areas, particularly in sports and battle. The other main possibility is Iris, the goddess of the rainbow, but Iris is usually depicted with a messenger staff, which this goddess does not have.