1A casual greeting used with people you know — 'hi', 'hey'.
やあ、元気だった？久しぶりだね。
Hey, how have you been? Long time no see.
やあ、今日はいい天気だね。散歩行かない？
Hi — it's nice weather today. Want to go for a walk?
廊下で彼を見かけて、思わず『やあ』と声をかけた。
I saw him in the hallway and couldn't help saying 'hey'.
2An exclamation expressing surprise or admiration — 'wow', 'oh'.
やあ、こんなところで会うなんて思わなかったよ！
Wow — I didn't expect to meet you here!
宝くじに当たったと聞いて、みんなが『やあ！』と驚いた。
When they heard about the lottery win, everyone exclaimed 'oh!' in surprise.
3A shout or yell used when exerting force or effort (like 'yah!' when lifting).
二人で重い机を持ち上げるとき、『やあっ！』と掛け声を上げた。
When two people lifted the heavy desk, they shouted 'yah!' together.