This is a libation, a sacrifice or offering of liquids.
The outpouring of liquids, libation, though it has now disappeared from our culture, was one of the most common sacral acts during prehistoric times and especially in the civilizations of the Bronze Age.
Walter Burkert (1985). Greek Religion, p. 70. Harvard University Press.
Burkert explains the purpose of the ritual as follows:
the libation of wine before drinking is a clear instance of a first fruit offering in its negative aspect. What is important is not that the libation reaches its destination, but that the offerer surrenders himself to a higher will in the act of serene wastefulness. The libations to the dead therefore signal a recognition of the power of the dead. What distinguishes the outpouring from other gifts of food is its irretrievability: what is spilled cannot be brought back. The libation is therefore the purest and highest form of renunciation.
Burkert, p. 72.
Libation vessels took many different forms, but in ancient Greece a common form was the φιάλη (phiale), a shallow dish with an indentation on the base to make it easier to grip when pouring out the offering.
Painted ceramic phiale with yellow and red dolphins and white octopuses. Eretria (Euboea), c. 510–500 BCE. In the Louvre, Paris. Photo by Jastrow.
Libation gives its name to Aeschylus’ play The Libation Bearers, in which Electra asks the chorus of handmaidens how she should make an offering to her murdered father Agamemnon:
You handmaidens who set our house in order, since you are here as my attendants in this rite of supplication, give me your counsel on this: what should I say while I pour these offerings of sorrow? How shall I find gracious words, how shall I entreat my father? Shall I say that I bring these offerings to a loved husband from a loving wife—from my own mother?† I do not have the assurance for that, nor do I know what I should say as I pour this libation onto my father’s tomb. Or shall I speak the words that men are accustomed to use: “To those who send these honors may he return benefits”—a gift, indeed, to match their evil? Or, in silence and dishonor, even as my father perished, shall I pour them out for the earth to drink and then retrace my steps, like one who carries refuse away from a rite, hurling the vessel from me with averted eyes?
Aeschylus (458 BCE). The Libation Bearers, 84–98. Translated by Herbert Weir Smyth (1926). Perseus Digital Library.
† This is sarcasm: Electra’s mother Klytemnestra murdered her father Agamemnon.
Libations were made to the gods as well as to the dead. For example, earlier in the Iliad, Achilles made a libation to Zeus before sending out Patroclus to fight the Trojans:
And in the front of all two warriors arrayed themselves for war, even Patroclus and Automedon, both of one mind, to war in the forefront of the Myrmidons. But Achilles went into his hut, and opened the lid of a chest, fair and richly-dight, that silver-footed Thetis had set on his ship for him to carry with him, whem she had filled it well with tunics, and cloaks to keep off the wind, and woollen rugs. Therein had he a fair-fashioned cup, wherefrom neither was any other man wont to drink the flaming wine, nor was he wont to pour drink offerings to any other of the gods save only to father Zeus. This cup he then took from the chest and cleansed it first with sulphur, and thereafter washed it in fair streams of water; and himself he washed his hands, and drew flaming wine. Then he made prayer, standing in the midst of the court, and poured forth the wine, looking up to heaven; and not unmarked was he of Zeus, that hurleth the thunderbolt.
Homer. Iliad XVI.219–232. Translated by Samuel Butler (1924). Perseus Digital Library.
The most detailed account of the act of libation is in Sophocles’ Oedipus at Colonus, where the chorus of elders of Colonus give Oedipus instructions for making a libation to appease the Eumenides (the Furies):
Chorus. Then make atonement to these divinities, to whom you have come first, and on whose ground you have trespassed.
Oedipus. With what rites? Instruct me, strangers.
Chorus. First, from an ever-flowing spring bring sacred drink-offerings, borne in ritually pure hands.
Oedipus. And when I have gotten this unmixed draught?
Chorus. There are bowls, the work of a skilled craftsman; crown their edges and the handles at either side.
Oedipus. With olive branches, or woollen cloths, or in what way?
Chorus. Take the freshly-shorn wool of a ewe-lamb.
Oedipus. Good; and then to what last rite shall I proceed?
Chorus. Pour the drink-offerings, with your face to the dawn.
Oedipus. Shall I pour them with these vessels of which you speak?
Chorus. Yes, in three streams; but the last vessel—
Oedipus. With what shall I fill this, before I set it down? Teach me this also.
Chorus. With water and honey; but add no wine.
Oedipus. And when the ground under the dark shade has drunk these?
Chorus. Three times lay on it nine branches of olive with both your hands, and meanwhile make this prayer.
Sophocles (c. 429 BCE). Oedipus at Colonus 469–484. Translated by Richard Jebb (1889). Perseus Digital Library.