What is a filter in CNN?

Filter is a window that scans the image. A filter is used to judge which feature do a pixel belong to. Does it belong to an arc , straight line, diagonal, etc. ?

One filter could be responsible for detecting arcs within an image, another could be responsible for diagonal lines, etc.

Filters -also known as kernels- scan the image region after another. The region size is determined by the window or kernel size.

Each filter has weights which characterise its function. A pixel type is judged by convolving the filter -centred on the pixel- with the image region.

Fortunately the weights of the filters are not set by engineers, they take their value by training.

They are filters, to filter the type of the pixel.