My real world function is on the color of passport and the meaning/status of the color. A function is a set of points in where every x-input have only one y-output. A relation is a set of ordered pairs.

The Independent Variable/Domain of my real world function is the color of the passport or the x-inputs.
The Dependent Variable/Range of my real world function is the status of color or the y-output.

The top graph represents a function in which every x-input have only one y-output and that it passes the vertical line test.
The bottom graph represents a relation in which every x-input have more than one y-input and it fails the vertical line test.