The trial (major plot spoilers for books 3 and 4)

Don't read on if you’re reading the series and want to preserve a moment of absolute jaw-drop. The picture doesn’t spoil anything by itself, so you’re safe there.

Adolin pulled free and crossed the stage. He supported Maya, putting her arm across his shoulder to hold her up as he would a wounded soldier. She clung to him, stumbling as she struggled to remain upright. Even as she did, however, she whispered it again.

“We chose,” she said, her voice ragged as if she had been shouting for hours. “Adolin, we chose.”

So, the explanation. =D I actually challenged my daughter to come up with a short one, but we both failed. Major plot/lore spoilers for Oathbringer and Rhythm of War, and maybe minor ones for Words of Radiance.

Thousands of years before the story proper, an order called the Knights Radiant protected humankind from evil. They bonded with spren (spirits), and had powers like flight or healing. For the more advanced, their spren could take the shape of Shardblades, living, powerful weapons that could take any shape their wielders wished (or whatever the spren felt was appropriate).

At one point in their history, lore says the Knights Radiant betrayed their oaths, thereby killing their spren. The sprens’ corpses remained frozen in the shapes of shardblades in the physical world, and became coveted, priceless items carried by kings and princes. The sprens' souls were scarred, catatonic beings with their eyes scratched out, called deadeyes, remaining in Shadesmar, the realm of spren, forced to become Blades whenever their new masters called for their weapons, incapable of thought or speech.

Present time: Prince Adolin possesses a Shardblade he's very attached to, knowing nothing of the history of Blades and spren. He even talks to his sword, which everybody around him finds weird. Over the course of the books, his Blade, partially awakened by the attachment he has for her, speaks to him for the first time, telling him only her name: Mayalaran, so he nicknames her Maya.

When he travels to Shadesmar to persuade the spren to bond a new generation of Radiants, the honourspren take him captive, and he offers to stand trial for the long-dead Radiants who betrayed their oaths and killed thousands of spren. The trial doesn’t go well, and seems lost when the prosecution calls Maya to witness - his own deadeye, whom he wields as his Shardblade in the physical world. The honourspren tribunal thinks Maya will be a silent witness to condemn him, but in that moment, by sheer force of will and her friendship for Adolin, Maya speaks, saying only two words which change everything. WE CHOSE.

Nobody ever wondered why the ancient Radiants betrayed their oaths and killed their bonded spren, which after all were practically part of their souls. Nobody ever wondered whether the spren had any agency over their fates. Nobody ever assumed that they actively decided, with their Radiants, that severing their bond was the lesser evil. And BOY, I can’t wait to find out, in the last book, what the bigger evil was that they averted.