CNN'S are what?

Question

I have a very fundamental question on what CNN'S actually are. I understand fully the training process as to take a bunch of images, start with random filters, convolve, activate, calculate loss, back propagate and learn weights. Fully understood....

But recently I came across this line on Slack

CNN'S can act as a frequency filter as well

for example, a blur is a low-pass filter and it can be implemented as convolution with fixed weights;

Please Explain? (Can't understand this at all)