The context for the photo is obvious from recent news, but the history of the phrase and the tradition was unknown to me. I found the following at Vinepair:
One of the oldest instances of pouring one out—technically known as making a libation—comes from Ancient Egypt, where the liquid offering for the dead was typically water (the rhythms of the Nile River being a source of life and death, that seems pretty apt). There’s even biblical... reference to the practice. Per Genesis 35:14, “Jacob set up a pillar in the place where he had spoken with him [God], even a pillar of stone. He poured out a drink offering on it and poured oil on it.” Not quite an offering for the dead so much as, well, Yahweh, but still, we have the concept of pouring liquid out as an act of reverence...And here's the etymology of the word "libation":
The Greeks had two kinds of libation, spondai and choai. Whereas choai were “poured out entirely and were used for libations to the gods of the underworld, the heroes and the dead,” spondai meant a “controlled outpouring of a small amount of liquid for the Olympian gods,” that liquid usually being wine.
Ancient Rome, unabashed copycat of Ancient Greece, also incorporated the practice of libation, both as an offering to the gods and as a means to honor the dead.
late 14c., "pouring out of wine in honor of a god," from Latin libationem (nominative libatio) "a drink-offering," noun of action from past participle stem of libare "pour out (an offering)," perhaps from PIE *lehi- "to pour out, drip" (source of Greek leibein "to pour, make a libation").
This is from an enlargement of the PIE root *lei- "to flow" (source also of Sanskrit riyati "to let run;" Greek aleison "a cup for wine, goblet;" Lithuanian lieju, lieti "to pour," lytus "rain;" Hittite lilai- "to let go;" Albanian lyse, lise "a stream;" Welsh lliant "a stream, a sea," llifo "to flow;" Old Irish lie "a flood;" Breton livad "inundation;" Gaelic lighe "a flood, overflow;" Gothic leithu "fruit wine;" Old Church Slavonic liti, lêju, Bulgarian leja "I pour;" Czech liti, leji, Old Polish lić "to pour"). Transferred sense of "liquid poured out to be drunk" is from 1751. .