「啦」（了＋啊 合音） "la" (le + a héyīn)
「啦」（了＋啊 合音）（"la" (le + a héyīn)）means “啦 = 了 + 啊 fused: marks a change while adding a lively, cheerful tone”. 啦 (neutral tone) is the spoken fusion of change-了 and softening-啊. Formation: 句子 + 啦（＝了＋啊，轻快语气）
啦 = 了 + 啊 fused: marks a change while adding a lively, cheerful tone
HSK4Formation
句子 + 啦（＝了＋啊，轻快语气）
Explanation
啦 (neutral tone) is the spoken fusion of change-了 and softening-啊. It does the job of 了 (a new situation) while 啊 layers on warmth and cheerfulness, so it sounds livelier and more casual than a bare 了. 好啦！= '(there,) okay now!'; 我走啦！= 'I'm off!'. Use it in friendly, upbeat speech; it is too casual for formal writing.
Examples
Note
啦 ≈ 了 + a cheerful lift. Colloquial; avoid in formal writing.
FAQ
「啦」（了＋啊 合音） — what does it mean and how is it used?
啦 = 了 + 啊 fused: marks a change while adding a lively, cheerful tone. 啦 (neutral tone) is the spoken fusion of change-了 and softening-啊. It does the job of 了 (a new situation) while 啊 layers on warmth and cheerfulness, so it sounds livelier and more casual than a bare 了. 好啦！= '(there,) okay now!'; 我走啦！= 'I'm off!'. Use it in friendly, upbeat speech; it is too casual for formal writing.
「啦」（了＋啊 合音） — how do you form it?
句子 + 啦（＝了＋啊，轻快语气）
「啦」（了＋啊 合音） — any usage notes?
啦 ≈ 了 + a cheerful lift. Colloquial; avoid in formal writing.