"Winter Is Coming" is the motto of House Stark, one of the Great Houses of Westeros. The meaning behind these words is one of warning and constant vigilance. The Starks, being the lords of the North, strive to always be prepared for the coming of winter, which hits their lands the hardest. The inevitable truth of the words are mentioned by other characters, such as MaesterAemon, who occasionally note that sooner or later, the Starks are always right. This was proven in The Winds of Winter, when autumn is ended with the coming of a white raven. Jon Snow remarks that his father, Eddard Stark, was bound to be right eventually.

A second meaning is that the Starks themselves are Winter, poetically, and that this is supposed to be an intimidating motto for the enemies of House Stark.

A third meaning, one that is long forgotten even by the Starks themselves, is that the White Walkers may return in the winter, and that the realm must always be prepared for such an event.

Behind the scenes

A deeper, metaphorical sense can also be found in the motto. According to George R.R. Martin, it more generally expresses the sentiment that there are always dark periods in each of our lives, and even if things are good now ("summer"), we must always be ready for a dark period when events turn against us ("winter"). In this sense "winter" parallels Richard of the House of York's opening line in Shakespeare's Richard III, "Now is the Winter of our Discontent / Made glorious Summer by this sun of York..."

In this sense it is loosely matched by the Latin phrase "memento mori" ("remember you have to die"), which was whispered into the ear of victorious Roman generals during their parade of triumph, to remind them that all earthly success is fleeting.

The phrase has transcended the dialogue of the show itself and is one of, if not the, defining quotes of the series in popular culture, along with "Valar Morghulis". It's frequently used on Game of Thrones merchandise, is often mentioned in reference to the show on social medias, and has found its way into the scripts of many modern-day TV programs and films. Recently, with winter arriving in the show, its prevalence has partially been replaced by "Winter is Here", though this phrase hasn't been uttered on the show with frequency.