A function is a relationship where each x-value is paired with exactly one y-value.
When looking at a graph, you want to ask yourself, "Can I find any two points on this graph that have the same x-value?"
I can see with x=8 that there would be two points on that graph with an x-value of 8, making this not a function.
People call this test "the vertical line test" because any vertical line really represents a single x-value. If any vertical line hits the graph more than once, the graph cannot be a function, because that value of x has more than one y-value.
