The Godot Event Manager is a tool designed for fast and easy design of game logic inside of maps. It can be used to quickly assign behaviour to parts of your map while also allowing a fine-grained control of them. Developers can also quickly create new building blocks to integrate their systems into the event manager.
Installing the addon is a simple matter of downloading and extracting a zip file. Please note that this also requires GDBasics and GDUIBasics in order to work. Both of which can be installed by downloading the latest zip file and extracting the content into your projects root folder. After installing them, finally download the latest file from here and also extract it into the project root. After that open your project settings (Project menu -> Project Settings…), switch to the Plugins tab and enable all three of the plugins. You might need to reload your project in order to make it work.