On verbs, it is used to indicate a yes–no question.
bIyaj. "You understand."
bIyaj'a'? "Do you understand?"
qagh SoplaH torgh. "Torg can eat gagh."
qagh SoplaH'a' torgh? "Can Torg eat gagh?"
On nouns, the suffix -'a' it has a different meaning, which will be introduced later in the course.
That should have a comma after "Mara", but Duolingo generally ignores punctuation.
That sentence is rejected because it unnecessarily changes the word order relative to the Klingon sentence.
We expect bIyaj'a', mara? to translate to "Did you understand, Mara?" and mara, bIyaj'a'? to translate to "Mara, did you understand?" -- that is, a vocative (indicating the name of the person you are speaking to) should be translated in the same position in the sentence as in the original.
Sometimes the word order has to be changed for grammatical reasons (to accommodate the different grammar of the two languages or the preferred style), but sometimes, multiple word order are equally possible, and in that case, the original word order should generally be followed -- this is the case with vocatives.