Zac and Phil discuss ABC deck building, which is a method of theorycrafting and deck building that helps players identify their strategy, make better card choices, and navigate games more effectively. They explain that ABC deck building consists of three components: A (the legacy power level thing to do), B (the enabler), and C (what is enabled). They provide examples of ABC deck building in various Legacy decks, such as Reanimator, Hogaak, and Delver. The hosts emphasize the importance of understanding the ABC plan in different matchups and how it contributes to a deck's overall power and flexibility.