Short version: Check the bottom link. The last vowel in a set of two words is prolonged to indicate that they're coordinated (it prolongs the vowel instead of using 'and')
Long version: There's a feature in Duolingo, and it took me forever to discover. On the website, you can actually access a little written chapter for each lesson. In the first chapter of High Valyrian they describe pronounciation, coordination and other useful aspects.
To me, this is crucial information for understanding a language rather than memorising it. I haven't a clue why it's not accessible from the app.
Anyway, here is the first chapter for High Valyrian :) https://www.duolingo.com/skill/hv/Basics-1/tips-and-notes