The range of a function is the set of numbers used as y-coordinates.
Let's look at the y-coordinates of the points in the graph. There are points in the graph with all y-coordinates greater than -3. There are points in the graph with all y-coordinates less then -3. At exactly -3, there is no point on the graph with that y-coordinate. We expect the graph to behave as it is shown above 7 and below -7, so the only value excluded from the y-coordinates is -3.
