First-class functions are a fundamental concept in programming languages where functions are treated as first-class citizens. This means that functions can be assigned to variables, passed as arguments to other functions, and returned from other functions, just like any other data type. This flexibility allows for more dynamic and modular code. In languages that support first-class functions, such as JavaScript and Python, developers can create higher-order functions that operate on other functions. This capability enables powerful programming techniques like callbacks, closures, and functional programming, enhancing code reusability and maintainability.