because it's the second person singular form of "sagon" (=to be), meaning "you are". cf. http://wiki.dothraki.org/High_Valyrian_Verb_Tables#sagon.
It's not stressed: It's long. Hopefully now that audio has been added you can hear the difference!
Stress is not marked in High Valyrian, because most of the time it's predictable. The only time is unpredictable is in commands (always stressed on the last syllable), and conjunctive lengthening (always stressed on the lengthened syllable).