I should mention, I (the owner of the site this is being hosted on, theki) did not create this nor did I contribute to it, like, at all. It is created by Nick Morgan (et al.) and is licensed under the GNU GPLv3 license. This merely exists as a way for me to access this at school.

What is this?

It's an emulator for the 6502 microprocessor. It's the one from Easy 6502 and supports all of the instructions on the original chip. Notes contains information about this implementation.
You should star the repository.