Here is an example. This image is the entire image, downsized to around 1000 pixels wide. That is about the widest we can post here (actually 1200 pixels but I did not deem it necessary to do that here). This is actually a print screen, in order to show you the crop boundaries of the next image.
Here is a "100% crop" of the lower left corner, the same "crop" as indicated by the crop rectangle in the image above. This image is 863 pixels wide. It is the same image you would see if you viewed my complete image at 100% zoom in a browser like ViewNX, or PhotoMechanic, which I used to prepare the crop.
The crop shows you the corner sharpness of this lens (20mm f/2.8 Ai-s at f/4) on my FX D700 body. This is a typical example of where someone might post a 100% crop to most accurately convey this aspect of that lens performance.