If a vertical line is placed at certain points on this graph, it will intersect the figure at more than one point. The definition of a function is a line where each input value of x has one and only one output value of y. This is where the vertical line test comes in.
In practice, this means that if a vertical line is placed on the figure and it intersects it at two points, the figure is not a function.
This figure fails this test, so it is not a function.