Syntax Meaning of a Class Name Followed by Curly Brackets in Swift

Question

Hi I'm a learner of Swift. I'm confused about the WindowGroup using in the following syntax. What does it do when we write a class name followed by curly brackets like the WindowGroup in following codes?

@main
struct MemorizeApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}