iMessage is a secure messaging service that you can use to send and receive messages in the Messages app on your iPhone, iPad, Mac, Apple Watch, or Apple Vision Pro.
If someone sends a message to your email address or phone number using iMessage, you receive the message send button; a green send button indicates the message will be sent with SMS, MMS, or RCS or your cellular service.
When you’re in a conversation with one other person, you can each see if someone’s writing a message. If the other person has undo send, group conversation management, and more.
When you use iMessage, you can Set up Messages on iPadApple Support article: What is the difference between iMessage, RCS, and SMS/MMS?