"Your dog is eating meat."
Translation:Twój pies je mięso.
Subject Pies, the thing/person that does somenthing is always in Mianownik/Nominative Direct object, the thing that somenthing is done to , after eat and many other verbs is in Biernik/Accusative.
As it happens, meat, that is eaten is neuter in polish which means it has Nominative=Accusative.
'twój' - singular you (you are talking to one person)
'wasz' - plural you (you are talking to two or more people)
In Duolingo sentences, in almost all sentences both will be equally correct, as you don't have the context. Only sometimes something else will point out to whether it's singular or plural.