Halcyon is a special kind of computer program that helps people play old video games. Imagine having a big box full of old toys that you really love, but some of them don't work anymore because they're too old. Halcyon is like a special machine that can fix those old toys so you can play with them again.
When you use Halcyon to play your favorite old video games, it creates a special kind of pretend computer that acts just like the old computer the game was made for. It's like building a special playground just for that one game to play in.
Halcyon helps the game and the pretend computer talk to each other so you can play the game like you used to a long time ago. So instead of having to throw away your old toys, you can use Halcyon to play with them again and feel like you're a kid again, even when you're all grown up.