Common Mob Interface
There are multiple mob frameworks which provide APIs for creating mobs. This is good since it gives modders a variety of different ways to make mobs, and one framework might be more suited to making a particular mob than another. But implicitly they also all provide their own API for "using" mobs: things like checking if an entity is a mob or doing things on mob death. This is bad because modders who want to interface with mobs created by other modders need to code to as many interfaces as there are mob frameworks. CMI is a mod that provides the "one true mob interface". It is designed to be extensible and easy to integrate.
Usage documentation is provided as LDoc comments in the source files. To get nicely-formatted docs, you can run ldoc on init.lua.
Implementing in your mob mod/framework
Part of the implementation of this interface is in the mods that support it. If you are a mob framework author or making a mob without a framework, and you want to support CMI, take a look at IMPLEMENTING.md.