The idea is to have a WYSIWYG view of your source code that makes use of standard and custom visualizations and editors to allow you to see and interact with data in a nicer way.
For example, for a game, you might build a custom level editor extension to Swift that allows you to code your levels in a nicer WYSIWYG view that is less prone to errors and generally nicer to deal with. Maybe you could even play your game in this view thus blurring the distinction between coding and execution.
P.S. Google “intentional programming”.