What's the difference between the keywords 'python' and 'python3' in command lines?

You can install multiple versions of python simultaneously in your system. Currently, python versions 2.x.x and 3.x.x is installed in your system.

Python3 is installed alongside with python(2) because some apps may still have a dependency on older python(2).

So python will refer to version 2.x.x and python3 will refer to version 3.

You can confirm the versions by:

python --version
python3 --version