What does pip install . (dot) mean?

I have a shell script whose last line is:

pip install .

What does it do?

• pip install <package-name> installs the specified package
• pip install -r requirements.txt installs all packages specified in requirements.txt

But I am not sure what the above command does.

"Install the project found in the current directory".

This is just a specific case of pip install /path/to-source/tree.

To quote the the pip install documentation describing this usage:

pip install [options] [-e] <local project path> ...