A redistribution of TeX Live, a typesetting environment which includes Mac-specific utilities and front-ends
MacTeX is a redistribution of TeX Live, a typesetting environment. While TeX Live is designed to be cross-platform (running on Unix, macOS, and Windows), MacTeX includes Mac-specific utilities and front-ends. It is also pre-configured to work out-of-the-box with macOS as it provides sensible defaults for configuration options that, in TeX Live, are left up to the user to allow for its cross-platform compatibility. - Wikipedia
See TeX Live.