Attributes provide more information about a declaration or type. There are two kinds of attributes in Swift, those that apply to declarations and those that apply to types.

You specify an attribute by writing the @ symbol followed by the attribute’s name and any arguments that the attribute accepts:

1. @attribute name
2. @attribute name(attribute arguments)

Some declaration attributes accept arguments that specify more information about the attribute and how it applies to a particular declaration. These attribute arguments are enclosed in parentheses, and their format is defined by the attribute they belong to.

1. @dynamicMemberLookup
2. struct DynamicStruct {
3. let dictionary = ["someDynamicMember": 325,
4. "someOtherMember": 787]
5. subscript(dynamicMember member: String) -> Int {
6. return dictionary[member] ?? 1054
7. }
8. }
9. let s = DynamicStruct()
11. // Using dynamic member lookup
12. let dynamic = s.someDynamicMember
13. print(dynamic)
14. // Prints "325"
16. // Calling the underlying subscript directly
17. let equivalent = s[dynamicMember: "someDynamicMember"]
18. print(dynamic == equivalent)

https://docs.swift.org/swift-book/ReferenceManual/Attributes.html#grammar_attribute-argument-clause