What does it mean to "Inherit global site-packages" in PyCharm?

The other answers aren't quite accurate. The "inherit global site-packages" doesn't "pre-install" or "add packages" into your virtual environment. The settings gives your virtual environment access to the global site-packages.

https://www.jetbrains.com/help/pycharm/creating-virtual-environment.html

Select the Inherit global site-packages checkbox if you want that all packages installed in the global Python on your machine to be added to the virtual environment you're going to create. This checkbox corresponds to the --system-site-packages option of the virtualenv tool.

https://virtualenv.pypa.io/en/16.7.9/reference.html

--system-site-packages Give the virtual environment access to the global site-packages.

It's just an option to pre-install some packages that you're using everytime, or if it doesn't bother you to have extra packages in your local python interpreted

• select it : all packages installed in the global python of your machine will be installed for the interpreter you're going to create in the virtualenv.
• do not select it : the interpreter you're going to create in the virtualenv will just have the basic, like pip, and setuptools, then you can install just what you need

Python global and venv :

• The global python, is the one in /usr/bin in Linux, or wherever in Windows, this is the main installation of the program, and you can add extra packages using pip

• When you're working on something, you may need only some packages, or specific version so not using the global Python. You can create a virtualenv, or pyenv, that will link a local python to the global one, for the main python functionnality, but the packages will be installed only in the virtualenv (and when using Pycharm, it can install for you the main package into the virtualenv you're creating)

PyCharm uses virtualenv to give your project its own set of packages that is separate from the rest of your system. “Inherit global site packages” means that the packages installed on your computer (outside of the virtual environment) will be added to the virtual environment.

You must understand what is inherit first. Inheritance allow you to use the method field of your parents. So in this case, you are allowed to use the package of your root python environment.