RFC 3066 is a document that defines a standard for language tags used in internet protocols. It specifies how to represent languages and their variations, such as dialects or regional differences, in a consistent format. This helps software and systems understand and process different languages correctly. The standard allows for the use of both primary language codes and optional subtags, making it flexible for various applications. For example, a tag like "en-US" indicates American English, while "fr-CA" represents Canadian French. This system enhances communication and content delivery across diverse languages on the internet.