XQuartz is an open-source version of the X.Org X server, a display server for the X Window System that runs on macOS. It allows cross-platform applications using X11 for the GUI to run on macOS, many of which are not specifically designed for macOS. XQuartz is a continuation of Apples deprecated X11.app which used to be part of OS X. It is required to run certain applications that are designed for Linux or Unix systems on a Mac. XQuartz is safe to install on a MacBook Pro.