Pixel-peeping means when you look so closely at an image (when you magnify the image so much) that the pixels are clearly visible. In that paragraph the intention is that when you upscale a photo, you generally do not want to see individual pixels - you want the image to be larger but still smooth. However, when you upscale an image for pixel-peeping, you want to see the individual pixels, and for that you use “nearest neighbor”.
I removed use of the term “pixel-peeping”, its use was not necessary to understand the concept.
I added an image to help explain the concept.
“detail” without “s” is correct.
“sharpening” removed.