#She is saying light em' up.
She knows she is about to die, and is basically telling Daenarys to "Burn the all" as the Mad king might say.

>This, as we all know by now, is the thing Dany says when she wants Drogon to breathe fire — the word literally means “dragonfire” in Valyrian. So why did Missandei say that? Drogon is nowhere to be seen in that scene, so surely she wasn’t trying to get him to light Cersei on fire or whatever. Right? Right. No, Missandei was just doing a badass sendoff. By choosing “dracarys” as her last word, she was basically telling Daenerys, “F— these guys.” Or as the Mad King might say, “Burn them all.”
Series creator David Benioff emphasized this point in the aftershow, phrasing it a little bit differently: “Missandei knows that her life is over, and she is saying, you know, ‘Light them up.'”

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<sub> [‘Game of Thrones': Here’s Why Missandei Said ‘Dracarys’ - TheWrap](https://www.thewrap.com/game-of-thrones-why-missandei-said-dracarys/)</sub>

So, she was just dissing Cersei (in a foreign language of course).