Interlace shows the image progressively from bad quality to full quality.
Without interlacing, pngs would load line by line pixels at full quality as the data is downloaded.
Interlacing would show the entire image at once, but VERY simplified in blocks of colors. Then as the data downloads, the blocks of color become smaller and more diverse, until the whole image is in its tiny pixel-sized boxes.
Personally, I like interlacing, but I always forget to do it! XD