It's just using the python match statement.

https://docs.python.org/3/tutorial/controlflow.html#match-st...

It's like switch in C / C++. And then it's just looking for the strings "while" or "end".

It's the same as if he had written:

    keyword = line.split(maxsplit=1)[0]
    if keyword == "while":
        ...
    elif keyword == "end":
        ...

The `pc` variable is the line number that is currently being evaluated.