What does it mean when the coefficient of the squared term is negative in regression?
In this context probably the plain English way to put it is that 'returns increase with additional exposure to the ad, but there is a tapering effect at the upper end of exposures. Looking at this picture of a parabola here (i.e a graph of y= -ax^2+bx+c):

Most likely your returns data are between 0 and the peak so you don't actually see  a decline, just a reduction in the rate of increase.