Metalanguage consists of a type of language that directly refers to the language itself used in communication.
The dictionary is a metalanguage example, as it has the purpose of describing and talking about the codes that make up the language itself. For example, when a person asks the meaning of a word, he is using the metalinguistic function (describe the language itself).
Metalanguage can still be applied in other areas, such as music, cinema, plastic arts, computer science, etc. In this case, the respective productions (as in music and cinema, for example) show elements that refer to the musical or cinematographic construction itself.
Following this reasoning, the self-portrait of a painter or photographer can also be considered an example of metalanguage. Another example of metalanguage in cinema is when characters "break the fourth wall", an expression used to represent the action of talking to the viewer.
Intertextuality and Metalanguage
As said, metalanguage is the language used to describe and explain language itself, in its most different styles (grammatical, artistic, musical, computer, etc.).
Intertextuality, on the other hand, refers to the set of references and relationships, implicit or explicit, that a given text can establish with other texts. In other words, it consists of a literary work, for example, which has influence and reference to other works, whether by the same author or by other authors.
There are different types of intertextuality, such as: title, quote, allusion, paraphrase and parody, for example.