In C programming language, a function is a self-contained block of code that performs a specific task or set of tasks. It can be called from other parts of the program to execute its instructions and return a value or perform an action. Functions help to modularize code, making it easier to read, understand, and maintain. In C, functions can be either built-in (such as printf() and scanf()) or user-defined, where the programmer defines the function's name, inputs, outputs, and logic. Function declaration, definition, and invocation are essential concepts in C programming, and mastering them is critical to writing efficient and effective code.