"Ēdrūs, taobus!"

Translation: Sleep, boy!

[SprightBark]

dothraki.org has a very concise section on the imperative: https://wiki.dothraki.org/High_Valyrian_Verb_Conjugation#Imperative

taobus is in the vocative case, used to address someone.

It's also worth a look at the use of the infinitive with negative, permissive, and third-person commands, noting that permissive commands are used with a preceding dative, and third-person commands are used with a preceding vocative: https://wiki.dothraki.org/High_Valyrian_Verb_Moods#Special_expressions