When you receive a message in email that can be answered with a short response, Outlook suggests three responses that you can use to reply with a couple of clicks.

When you use the Suggested Replies feature, Outlook uses a machine learning model to continually improve the accuracy of the suggestions. This model runs on the same servers as your mailbox within your organization. No message content is transmitted or stored outside of your organization. 

Learn how to use or disable this feature: Use suggested replies in Outlook (microsoft.com)