First, recall that a function is a relation between two sets, with the special condition that for the domain, it doesn't assign different values for the same input. This fact translates to the fact that if you have a graph, and the graph is the graph of a function, then if you draw a vertical line on any point of the domain, the line would cross the graph only at one point. This is called the vertical line test. If the line crosses two points, then the graph is not a function.
Consider in our case the following graph.
If we draw the following vertical line:
we see that the line touches the graph in two different points. So the graph is not a function.