The API is simply a group of functions (code) in pocketmine that are designed for plugins to use to interact with the server, players etc. In its words the API is simply that part of the pocketmine code that you can use in plugin development. If you mean 'which API', that is displayed when you start the server...
Either look at the source code directly: https://github.com/pmmp/PocketMine-MP Or look at that automatically generated one (which's design hurts my eyes lol): https://jenkins.pmmp.io/job/PocketMine-MP-doc/doxygen/ P.S.: An IDE can be helpful when developing plugins, because it will autocomplete functions etc.
create new project and then right click on 'External Libraries' then configure php include paths and add pocketmine source code