what is iMessage

iMessage is Apple's built-in instant messaging (IM) service. It lets you send text, picture, video, sound, and location quickly and easily to anyone else using iMessage on iPhone, iPad, Mac, or Apple Watch.

You can sign up for this on your Settings>Messages then turn the button on

This should help explain.

Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support

Hi iMessage is like a SMS text message but iMessage is just for Apple devicesa from one apple device to another apple device.

If you send to an Android device it will just go as a text message.

Cheers

Brian 🇬🇧