Good architecture is not code that works; it is code that can be extended, tested and maintained without a rewrite. The piece takes Robert C. Martin's five principles and treats them as tools for specific problems: lowering coupling so a change stays local, reducing the risk that a modification breaks something distant, and making a system straightforward to test and automate.
Each principle — single responsibility, open/closed, Liskov substitution, interface segregation and dependency inversion — is worked through with Swift examples rather than left as an acronym to recite.
Every section follows the same shape: a small, plausible piece of Swift that already violates the principle, an explanation of why that shape causes trouble as the code grows, and a revised version that fixes it. The violations are ordinary ones — a model object that also sends email, a payment function that grows another if-branch per provider, a subclass that can't honestly fulfil its parent's contract, an interface that forces a type to implement behaviour it doesn't have, a service wired directly to one database implementation.
The Liskov and interface-segregation examples sit next to each other on purpose, since both are about a type being asked to support more than it should — one through inheritance, the other through an interface — and the piece draws that line explicitly rather than leaving the two principles to blur together.