The action server is a rest service that responds to the calls made by Rasa Core. You tell the core where to find your service via the endpoints.yml config file. The setting most used is this:
action_endpoint:
url: "http://localhost:5055/webhook"
You would setup a REST server in C# or Java and it needs to respond to the endpoints that Rasa will call.
There’s documentation on Core’s request to the action server here.
If you want to implement forms, please note the recent discussion here