ANTLR is a powerful parser generator that is used to build language recognizers, interpreters, compilers, and other tools for transforming structured text. It relies on a grammar specification to generate code that can recognize and parse input according to that grammar. ANTLR supports multiple target languages and can be used to develop applications in a variety of domains, including database management, software engineering, and natural language processing. Proficiency in ANTLR requires knowledge of parsing theory, formal grammars, and programming languages.