Basically you are specifying the location from which the pip package manager to extract the package information from.
E.g.:

# Get the Source code
git clone 

# Change into the cloned git repository:pytorch-fcn
cd pytorch-fcn

# Install the package definitions from current location i.e. pytorch-fcn.
pip install .

Here is a brief description: pip executes the setup.py which loads the requirements.txt which has textual representation of the dependency packages.

References:

1. Available here is the official documentation of pip.
2. For better python development workflow refer here for why you should use pip and virtualenv.