Fraunhofer diffraction and convolution of two aperture functions

Hello,

I'm having some trouble understanding the concept of two function convolution in Fraunhofer diffraction.

Let's say I have an aperture function in the shape of some cosine function (which is always above zero), and I want to calculate the transmission function if I only illuminate 3 such "slits" (so I capture 3 peaks of cosine aperture function). In order to do that, I was told to take the convolution of two Fourier transforms: the transmission function of cosine aperture illuminated over infinite slits and the transmission function of the Rect. function (some kind of box, which captures the 3 peaks of cosine function), which is a Sinc. function.

Is this correct? I don't quite get the physical meaning of convolution in this particular case.

Thanks for any explanations.