XQuartz is an open-source implementation of the X Window System for macOS. Essentially, it lets you run X11 applications (programs typically found on Linux and Unix-like systems) on your Mac.

Think of it as a bridge. Without XQuartz, many X11 applications wouldn[SQ]t work on macOS. It provides the necessary environment and libraries for these applications to function correctly. So if you need to use a scientific visualization tool, a specific development tool, or other software that relies on the X Window System, XQuartz makes it possible.