A type that can be initialized using a dictionary literal.
Swift Standard Library
A dictionary literal is a simple way of writing a list of key-value pairs. You write each key-value pair with a colon (:) separating the key and the value. The dictionary literal is made up of one or more key-value pairs, separated by commas and surrounded with square brackets.
To declare a dictionary, assign a dictionary literal to a variable or constant:
When the context provides enough type information, you can use a special form of the dictionary literal, square brackets surrounding a single colon, to initialize an empty dictionary.
Conforming to the ExpressibleByDictionaryLiteral Protocol
To add the capability to be initialized with a dictionary literal to your own custom types, declare an init(dictionaryLiteral:) initializer. The following example shows the dictionary literal initializer for a hypothetical CountedSet type, which uses setlike semantics while keeping track of the count for duplicate elements: