A function is a relationship with a quantity (number). For example, if you made a table, and had one side be 1 2 3 4 5 (X axis) and then the y axis (other side) be 5, 10, 15, 20, 25, the function (rule) would be y= x5. The vertical line test is used to see if a graph has a function or not, so if it creates a vertical line on a graph, it has a function.
A real life situation would be miles per hour, so, for the x axis (hours) it said 1, and for the y axis (miles) it would be 60, this means in a duration of 1 hour you went 60 miles. Well, lets say you went 2 hours driving, the miles would be 120.
For 3 hours, it would be 180. The function here is y= x60!
If you have any questions ask me!