please explain how exactly this program is working, i unable to understand the logic of the matrix( kernel) ,

secondly how can i know the location of the edge...thank you 

/**
* Edge Detection.
*
* Exposing areas of contrast within an image
* by processing it through a high-pass filter.
*/

float[][] kernel = { { -1, -1, -1 },
{ -1, 9, -1 },
{ -1, -1, -1 } };

size(200, 200);
PImage img = loadImage("house.jpg"); // Load the original image
image(img, 0, 0); // Displays the image from point (0,0)
img.loadPixels();
// Create an opaque image of the same size as the original
PImage edgeImg = createImage(img.width, img.height, RGB);
// Loop through every pixel in the image.
for (int y = 1; y < img.height-1; y++) { // Skip top and bottom edges
for (int x = 1; x < img.width-1; x++) { // Skip left and right edges
float sum = 0; // Kernel sum for this pixel
for (int ky = -1; ky <= 1; ky++) {
for (int kx = -1; kx <= 1; kx++) {
// Calculate the adjacent pixel for this kernel point
int pos = (y + ky)*img.width + (x + kx);
// Image is grayscale, red/green/blue are identical
float val = red(img.pixels[pos]);
// Multiply adjacent pixels based on the kernel values
sum += kernel[ky+1][kx+1] * val;
}
}
// For this pixel in the new image, set the gray value
// based on the sum from the kernel
edgeImg.pixels[y*img.width + x] = color(sum);
}
}
// State that there are changes to edgeImg.pixels[]
edgeImg.updatePixels();
image(edgeImg, 100, 0); // Draw the new image