What do square brackets mean in pip install?

Question

I see more and more commands like this:

$ pip install \"splinter[django]\"

What do these square brackets do?

Answer 1

This is exactly the list from the setup.py file for the project in question:

"django": ["Django>=1.7.11;python_version<'3.0'", "Django>=2.0.6;python_version>'3.3'", "lxml>=2.3.6", "cssselect", "six"],