iMessage is a secure messaging service that you can use to send and receive messages on your Mac or iOS device.
If someone sends a message to your email address or phone number using iMessage, you receive the message on all your devices that are set up to receive messages sent to that email address or phone number. When you view an iMessage conversation, you see all messages sent from a Mac or from an iOS device, so you can talk to your friends wherever you are. See the Apple Support article Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch.
You need an Apple ID to use iMessage. If you’ve made purchases from the iTunes Store or App Store or you’ve signed in to iCloud, you have an Apple ID. If you don’t have an Apple ID, you can create one in Messages.