The Script action will execute a script written in C#, a structured programming language that uses the .Net runtime.
Clicking the script preview area will bring up a script builder with syntax highlighting.
However, it's probably better to write your scripts using Visual Studio Code, or Visual Studio Community to write your script and test it, then paste it in. You will have a much better editing and debugging experience this way.
If you are going this route, it's probably relevant to you that VoiceWarrior uses the CSScript library to execute scripts.