What is CNN (Convolutional Neural Network)?

In digital image processing, we use convolution to filter, because the convolution template we use is actually a Qualcomm low-pass bandpass filter in frequency domain. But in the neural network, the parameters of the template are trained, which I think is purely mathematical, and it is difficult to understand its significance in frequency domain, so I think the convolution in the neural network has no filtering effect. Then talk about personal understanding. First of all, whether it is a convolutional neural network or not, as long as it is a neural network, it essentially uses a simple function to fit an extremely complex function (whether it is sigmoid or Relu), and the fitting process is to adjust the parameters through repeated back propagation to minimize the cost function.