The ModAPI makes it possible to modify managed game assemblies. In order to do so it uses Mono.Cecil (it rocks!). The code is currently not documented but it will get a full documentation in the future. This early release to open source was done mainly because some people claimed this software does contain viruses. The benefit of open source is not only to get awesome people work on something great, but also to ensure that other people can confirm there is no harmful code.
If you are interested in example mods you can lookup the following repository: https://github.com/FluffyFishGames/ModAPI-The-Forest-Mods
All out of the box mods are included in this repository to give you an insight of how mod creation works.