Originally titled "Babricated Forge", Apron is a mod for Minecraft Beta 1.7.3 that adds compatibility with the mods of the era (and those that were made by some very short-sighted developers later on) to the Fabric modding environment.
Apron was originally intended to only offer a compatibility layer with API mods of the time. This would have allowed mods to interact with the APIs themselves, and nothing else! Not even the game itself would have been accessible.
Of course, this would not have helped with anything by itself.
So, soon enough, Cat Core joined the project and offered help in the form of Mod Remapping API integration.
MRAPI is a mod that can help other mods function in different mappings by translating calls using obfuscated names into calls using any other name, like in our case: Fabric intermediary names!
If you'd like to chat about Apron (or indeed any other Minecraft compatibility layer), join the Fabric Compatibility Layers Discord!