It's used as an indicator - think of it as "to". You learn about it in the first skill of the Hawaiian course, where it's said as "e". So you would say "Aloha, e Kawika" which is "Hello to Kawika". That's how I learnt how it works. Please, someone feel free to correct me if I'm mistaken,
Click on Phrasas and before START click on the lightbulb, there is an explanation: In Arabic, you use the word يا (yaa) before addressing someone. You can think of it as an attention getter, kind of like “hey!” but not as informal.
أَهْلاً يا عُمَر!
شُكْراً يا كَري!
Thank you, Carrie.