Functions are a kind of relation. In a function, we have that for each input we only will have an output. In this kind of relation, we have that, for every value of the independent variable, we have only one value for the dependent variable.
When we have a graph of the form:
We can determine if we have a function applying the vertical line test. If we draw a vertical line to the graph, and the line crosses the graph of the relation in more than one point, then we have that the relation is NOT a function:
Since the blue line (vertical line) crosses the graph in two different points, we can conclude that the relation is NOT a function because we have two different outputs for one input.