Compilers perform a series of tasks including lexical analysis, syntax analysis, semantic analysis, optimization, and code generation. Lexical analysis involves breaking down the source code into meaningful tokens. Syntax analysis checks the tokens against the grammatical rules of the programming language. Semantic analysis ensures the tokens have valid meanings according to the language's definitions. During optimization, the compiler improves the code to run more efficiently in terms of performance and resource usage. Finally, code generation creates the output in a target language, which can be machine code or an intermediate representation for a virtual machine.