What is this?

The short answer:

This is a website that runs a python-script in HTML that lets you convert midi files (a file format for songs and music, similar to MP3) into a text based format for 2 rhythm games.

The long answer:

This is a website built by brianops1 using google sites, that uses an embedding system called py-script (https://pyscript.net/) that lets you run python scripts in HTML.

The original Python program was made for Piano Visualizations V.1, a roblox game that has a separate embedded game called "Piano Mania" which is a rhythm game integration for Piano Visualizations.

However, since it was made with python, I didn't want to go through the effort of making it a py.exe or try to make it cross platform, so I converted that program into a website.

As for the difference between Midi Converter 61 and Midi Converter 72, the Piano Visualizations game uses a format similar to Virtual Piano and thus only accepts 61 keys.
However, a new game that I'm currently developing (or is finished depending on when this is being read) uses a 72 key format instead. And thus why there are 2 different converters.