Answer: SOLID is a set of five design guidelines for maintainable OOP code:

Principle	Idea	Swift application
Single Responsibility	A type should have one reason to change	Split a "God ViewController" into a ViewModel + Service
Open/Closed	Open for extension, closed for modification	Add behavior via protocol conformance/extensions, not by editing existing types
Liskov Substitution	Subtypes must be usable wherever their base type is expected	A Square: Rectangle shouldn't break callers that resize rectangles independently
Interface Segregation	Prefer many small protocols over one large one	Codable is Encodable & Decodable — adopt only what you need
Dependency Inversion	Depend on abstractions, not concrete types	Inject a NetworkClientProtocol, not a concrete URLSessionClient

// Interface Segregation + Dependency Inversion together
protocol UserFetching {
    func fetchUser(id: String) async throws -> User
}

final class ProfileViewModel {
    private let fetcher: UserFetching   // depends on an abstraction

    init(fetcher: UserFetching) {
        self.fetcher = fetcher
    }
}