A lexer, short for lexical analyzer, is a program that processes input text to break it down into meaningful components called tokens. These tokens represent the basic building blocks of a programming language, such as keywords, operators, and identifiers. By identifying these elements, a lexer helps facilitate the understanding of the code structure for further processing. In the context of programming languages, lexers are often used in conjunction with parsers, which analyze the tokens generated by the lexer to build a syntax tree. This process is essential in tools like compilers and interpreters, enabling them to translate high-level code into machine-readable instructions.