Last night I was using my iMac when a text message came through on the Messages app. Nothing odd about that until I realised the sender was an Android user. The text was in green. This is a High Sierra feature. No, it isn't. It's been there since Yosemite.
If you set your iPhone to forward text messages, any iOS 8.1 (or newer) or macOS 10.10 or newer device that’s using the same Apple ID in the Messages app can send and receive SMS & MMS messages in addition to iMessages. Exceptionally useful.