To illustrate scope, let's look at some simple code:

var outer = "Hello"
if outer == "Hello" {
    var inner = "World"
    print(outer)
    print(inner)
}
print(outer)
print(inner) // Error: Use of unresolved identifier 'inner'

As you can see, outer can be accessed from both in and out of the if statement. However, since inner was defined in the curly brackets of the if statement, it cannot be accessed from outside of them. This is true of structs, classes, loops, functions, and any other structure that involves curly brackets. Everything that is not in curly brackets is considered to be at global scope, meaning that anything can access it.

Sometimes, it is useful to control scope yourself. To do this, you can define types within other types:

class OuterClass {
    struct InnerStruct {
    }
}

In this scenario, InnerStruct is only directly visible from within OuterClass. This, however, provides a special scenario that is not there for other control structures like if statements and loops. If code at the global scope wanted to access InnerStruct, it could only do so through OuterClass which it does have direct access to, as shown:

var inner = OuterClass.InnerStruct()

This can be useful to better segment your code but it is also great for hiding code that is not useful to any code outside other code. As you program in bigger projects, you will start to rely on Xcode's autocomplete feature more and more. In big code bases, autocomplete offers a lot of options, and nesting types into other types is a great way to reduce unnecessary clutter in the autocomplete list.