What is the calculation done when going from an input of a higher number of channels to fewer channels in a one by one convolution?

if you have a in input of 28 x 28 x 48 (48 being the number of channels) and you convolute using a 1 x 1 x 16 filter, what exactly is the multiplication that happens that would result in a 1 x 1 x 16 output for each implementation of the filter?

I understand that this would be applied 28 x 28 times. But not clear on what happens on each application of the filter.