Help is needed to learn the article Code History: Creating the Game Model

Hi, I would like to understand the topic of Swift Development in one of the articles.
https://www.codecademy.com/paths/ios-developer/tracks/building-code-history-part-two-swiftui/modules/building-the-game-model-swiftui/articles/creating-the-game-model

I am having difficulty of understanding the code line to line.