This is the 5831st most frequent Chinese word.
回信
Reply to a letter or message.
In this sentence, '回信' is used as a verb, meaning 'to reply to a letter'. It indicates an action of responding to a written communication.
我很快会回信给你，请稍等。
I will reply to your letter soon, please wait.
Here, '回信' is used as a noun, referring to the 'reply letter' that someone receives.
他昨天收到了你的回信，非常高兴。
He received your letter yesterday and was very happy.
In this context, '回信' is again a verb in imperative form, suggesting the action of replying with details about a topic.