Convolution.

⊗ and ⊕ denote multiplication and addition, respectively. Some operations are grayed out just for visibility. A patch of the input image (whose size is the same as that of the kernel) is multiplied with the kernel pixelwisely, and then summed up into a single pixel of the output image. In reality, the input image may be a single channel of a multi-channel input image; then, multiple intermediate output images are produced with different kernel channels, and then summed into a single output channel of a final output image.