The document discusses several important concepts in digital image processing including: 1. The relationships between pixels in an image including neighbors, adjacency, connectivity, and distance measures. Pixels have 4-neighbors, 8-neighbors, and diagonal neighbors that are used to define adjacency. 2. Regions in an image are connected subsets of pixels, while boundaries are pixels on the edge of a region that are not in the region. 3. Common distance measures between pixels include Euclidean, city-block (D4), and chessboard (D8) distances that measure distances as radii of circles/squares centered on reference pixels.