what does 야 mean exactly? Wiktionary says it is a vocative case marker, but what does that really mean?
while ~아/야 are vocative markers (calling someone by name), this construction is part of the ~어/아야 construction, which is well explained in the following page: Lesson 137: ~아/어야: One must do an action in order to do another
A vocative marker means that you’re directly addressing someone. For example, if you were to call someone by name in informal speech, you would say “경미야” or “바름아” rather than just the name. For these markers, if the name ends in a vowel, you use “야” and after a consonant you use “아.”
English usually indicates these direct addresses by putting more stress on someone’s name and lowering the intonation, but I doubt you’ll actually see that in an English grammar book. You’ll notice the vocative case a lot more in languages that decline their nouns, like Greek and Latin.