Apple HIG means Apple's Human Interface Guidelines: design guidance for making apps for Apple platforms. It covers ideas such as clear navigation, readable content, familiar controls, and accessibility.
Think of it as a guidebook for building an app that feels understandable to people using an iPhone, iPad, Mac, or another Apple device. It can help a team make choices that fit the platform.
The guidelines are not a rule that every screen must look identical, and they do not replace learning what a product's users need. Teams should use them as guidance and check their design in context.