Here we have the graph of y=ax2 with four different values for a.
Notice that when a is a positive number the parabola points upwards, or in a positive direction; when a is a negative number, the parabola points downwards, or in a negative direction.
These examples show the variation when a changes from 1 or - 1 to 10 or -10. We can see that in the case when a=10 or -10 the parabola is much closer to the x-axis; the curve of the parabola is shrinking.