In Game of Thrones S8 E04, "The last of the Starks", Cersei kills Missandei, before killing she asks if Missandei has any last words, and Missandei replies
Why did she say that, but nothing else?
We all know Dracarys is the word to release Dragon fire. Missandei is telling Daenerys to burn Cersei Lannister.
Nathalie Emmanuel who played Missandei tweeted after the release and response of S08E04. She tweeted "Dracarys" and replied with "Burn the bish". Bish is slang for bitch and she is referring to Cersei Lannister by that word.
She knows she is about to die, and is basically telling Daenerys to "Burn them all" as the Mad King might say.
Dracarys is clearly meant for Dany. Missandei knows that her life is over, and she is saying, you know, "Light them up."
In addition, she herself later confirmed this in an interview with elle.com.
(NATHALIE) She's normally quite calm and stoic. Her yelling is not really something we've seen before. I wanted to make the most of this moment and make sure that it meant something. In a way it was her own war cry, to her friend and her queen. It's so funny because she'd told Grey Worm earlier, "My people are peaceful." But in her last breath she's like, "Burn them all! Burn these fuckers to the ground."
Dracarys means dragonfire and is what Dany says to command her dragons to breath fire. She is telling Dany to burn Cersei, and burn anything that stands in her way.
Missandei knows she's not leaving the rampart alive, and, rather than pleading or showing weakness, she stands tall and shows defiance.
(This is roughly analogous to "Give me liberty or give me death," although Patrick Henry was not facing execution when he said it.)