A graph is a function if it passes the vertical line test. The vertical line test is where you take a vertical (up and down) line and test all spots of the graph. If the graph intersects the vertical line more then once then it is not a function, it is a relation. If the vertical line only crosses the graph once then it is a function.
Look at the image below to see the vertical line test for this graph:
The vertical line crosses the graph twice, therefore this is not a function. It is a relation.
Hope this helped!
~Just a girl in love with Shawn Mendes