argparse is used to call your script with arguments. Here is a small example:
python imagenet_eval.py --data /home/petro_key/python/data -j 12 -b 64

As you can see some arguments are defined with a valid default value, e.g. the learning rate, while other arguments need to be set like data.
Basically argparse tries to parse all set arguments you pass to your script and sets the corresponding values.
It’s comparable with char *argv[] in C++, if you are familiar with it.