Oh. Meant to add that it's also a depiction of the tree upon which the Children of the Forest made the first White Walker (show verse*). Symbol was there too. (Also they made the same symbol at the Fist of the First Men. Remember Jon, Mance, Tormund, Orrell, and Ygritte's scene in Season 03?)
*Book verse, we must ask ourselves why they're called both "Children of the Forest" (children have parents; the Mother is in the Wood) and "those who sing the song of earth and stone" (the Mother in the Wood is also the earth and stone--the earth deity--whose song they "sing" being, of course, also "the song of ice and fire"). $0.02.
Edit: Just watched the scene again and, although it isn't a perfect parallel of Azor Ahai slaying Nissa Nissa and the cycle of Trios the Three-Headed Dragon/Three-Eyed Crow/Sphinx triune deity (the sun slaying the moon, but also the sun slaying the earth, since she's both), you even see Azor Ahai/Last Hero figure Beric Dondarrion slay the Umber child wight with his flaming sword--the red comet). So, reiterating again the Winter Sun/Azor Ahai doing his icky villainous thing.
However, as J. Stargaryen mentions, the Winter Sun is also a symbol of death (the seasons of the sun corresponding to the other aspects of the deity -- Spring/birth, Summer/growth, Autumn/decay/aging, Winter/death) and the sun/son play on words, you're brought back to Azor Ahai's/the Father's slaying of his own son (who is the sun's son) and solar figure himself, the Green Giant (the earth--earth being mother followed by son; queen followed by prince; that is: king's blood to wake the stone dragon, the (mother) first and then the son, so both die kings(regents), etc.) and thereby we do indeed see that double symbol of winter sun accounts for both the murderer and his victim (the father first and then the son, so both die kings--which is sort of fudged in that the father kills his son but then the mother--the ghost weeping tears of blood crying out from the grave for a son to avenge her--resurrects him to kill the father, the reverse of the situation with the stone dragon being "born with the dead" like the other Mother/Son symbol, the direwolf pups, are.).
And that brings us to Last Hearth and their sigil (giant breaking free of his chains: the green giant, the stone dragon, the prince that was promised, breaking free of his mother's chains--as he's "a puppet dancing on a string," the wight carrying out her bidding, turning against her to slay her--Winter--as well, becoming THE LAST HERO). A hearth is a place where we burn wood to keep warm, of course, so the sun's son takes upon his fiery persona of the deity (becoming, in effect, a burning tree himself as well as a burning sword) to burn the trees (white and black both, symbols of the mother), slay his own mother, and bring back the sun--spring--dawn. As Tywin likes to say of one of our six dragon children, Tyrion: you, who killed your mother to come into the world!
There's so much reiteration of symbols, I could go on forever, but that would be cruel, so I won't. Lol.
