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"). Let's say you want to know how many candies you'll get if you give a certain number of coins to a candy machine. The candy machine takes in a number of coins, and then gives you a different number of candies. In this case, the number of coins is the input and the number of candies is the output.
So, in a function, you have an input and an output, and there's a rule that tells you exactly what to do to get the output from the input. It's like a recipe, and no matter what input you use, you always get the same output following the same rule.
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.