From their “what’s this?” page:

What is this?

This site is dedicated to resources on video game decompilation. Matching decompilation is the process of writing source code (typically C or C++) that compiles to the same exact binary as the original. This usually requires using the same compiler as the original game, with all source code being written from scratch. Because matching decompilation is complex and time-consuming, dedicated communities have formed around the challenge. These communities have built a wealth of knowledge and tools to support the process.