Built-in functions are pre-defined functions provided by programming languages that perform specific tasks. They simplify coding by allowing developers to use these functions without needing to write the underlying code. Examples include functions for mathematical operations, string manipulation, and data type conversion, which can save time and reduce errors. In languages like Python, JavaScript, and Java, built-in functions enhance productivity and code readability. They are part of the language's standard library, making them readily available for use. Understanding and utilizing these functions is essential for efficient programming and can help beginners learn coding concepts more easily.