The coefficient produces a transformation called a reflection in the x-axis whenever , that is, this axis is said to be a mirror. Let's take an example, suppose you have a function:
If the leading coefficient is now -4, then the graph is reflected and the new function is:
This is shown in the figure below. The red parabola represents and the blue parabola is . As you can see, the red parabola has been reflected and transformed into the blue one.