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. The lexer scans the source code and categorizes each part, making it easier for the next stage of processing, typically a parser, to understand the structure of the code. In the context of programming languages like Python or Java, a lexer plays a crucial role in the compilation or interpretation process. By converting raw text into tokens, it helps ensure that the code can be analyzed and executed correctly. This step is essential for creating software that runs efficiently and accurately.