A function is like a machine that takes in a number (or an "input"), does something to it, and gives you another number (or an "output"). A function is a mathematical concept that maps a set of inputs, called the domain, to a set of outputs, called the range. It defines a unique output for each input in the domain. The relationship between the inputs and outputs is often described using an equation or a set of rules. In mathematical notation, a function is typically denoted by f(x), where x is an input and f(x) is the corresponding output. Functions can be used to model real-world situations and can help to simplify complex problems.