This is part of the “Continuity” feature added in Mac OS X 10.10 Yosemite. The built-in SMS-message-sending feature on a Mac only works if you partner it with an iPhone, too - not if you have an Android phone. You need both - there’s no way to do this with an iPhone and a Windows PC. If you have both an iPhone and a Mac, Apple makes this possible.
RELATED: How to Make Macs and iOS Devices Work Together Seamlessly with Continuity
