A function is a relationship in math between two variables, often x and y, and for every value of x there is exactly one value of y. The x value is referred to as the independent variable and the y as the dependent variable.
The term f(x) is often used to represent a function. One example of a function is: f(x) = x + 2. This means that for each value of x, f(x), also know as the y value, is "x + 2". The value of y depends on the value of x. A graph is a function if it passes something called the vertical line test. If a vertical line can be drawn at any point and only pass through the graph once, then the graph is a function.