I wrote the original version of this "simple web server" app (https://chromewebstore.google.com/detail/web-server-for-chro...) because the built-in python http server is a bit buggy. It would hang on some connections when loading a webpage with a lot of small assets.
I was surprised how many people found it useful. More so when Chrome web apps were supported on other platforms (mac/linux/windows).
127.0.0.1 means "self" in IP. Presumably that means that it if you browse to your IP address from your computer it will work, but from your phone it will not.
I usually do the opposite - 0.0.0.0 - which allows connections from any device.
Python 2 had a similar function (`python -m SimpleHTTPServer`). I know there's a Python 3 equivalent, but I don't have it memorized.