Convolution is explained and vividly visualized by 3b1b so clearly. it’s two functions convoluted/combined.

f * g = f(x) g(s-x)

xxx is the position where we evaluate the result of the convolution.sss is a dummy variable of integration (or summation in discrete cases), representing different points in the domain of fff. Think of sss as a “running index” that helps scan through the function fff.