Jon Snow finally got to the dragonglass stores beneath Dragonstone and found some pretty interesting artwork while he was in the caves. So what do the cave drawings mean on Game of Thrones? It's pivotal to Jon winning his argument with Daenerys.
Thanks to a super handy tip from Samwell Tarly, studiously poring over the tomes in the Citadel as a maester-in-training, Jon Snow learned that there was dragonglass in the caves underneath Daenerys' ancestral seat, just begging to be mined and turned into weapons to fight the Army of the Dead. (Remember that dragonglass is one of the few things that can destroy a White Walker.) But first, Jon had to convince his aunt to let him access it. Eventually, as a show of good faith and in hopes of gaining him as an ally against Cersei Lannister, Dany acquiesced to Jon's request, although his refusal to acknowledge her as Queen of his kingdom in the North is causing some tension.
She doesn't exactly believe Jon when he says he's too preoccupied with defeating an army of dead men to focus on humans fighting other humans. Enter: ancient cave drawings in the dragonglass mines that prove his point precisely. Before Jon and his men start tearing up the caves, he decided to show Daenerys around, just so she can see what she's sitting on.
In addition to the beauty of shining volcanic glass sparking in the stone walls as far as the eye can see, Jon spotted some cave drawings made by the Children of the Forest. Remember that they were the non-human race who first lived in Westeros, which was eventually colonized by the First Men. When Jon first shows Dany the drawings, she mistakenly believes them to have been drawn before men first came to Dragonstone. But the drawings clearly depict both the First Men and White Walkers. Jon uses these drawings to show Dany that the Children of the Forest and the First Men lived together and joined forces against the White Walkers, understanding that their differences meant nothing against a common, deadly enemy.
Needless to say, Daenerys finally came around and got a jump start on killing off Cersei's last lines of defense while Jon mined the caves. She obliterates Jaime's army, the men who were curiously absent from last week's slaughter at Casterly Rock, which hopefully means she's speeding up her world takeover to join Jon at his side against the real enemy.