That is when you open your photo in Photoshop, zoom to 100% (which usually means you will not be able to see the entire shot, only a portion of it), then using the rectangle selection tool, select an area, then crop away every thing other than the selection. For the purpose of posting here, you want the selection to have a longest dimension of no more than 800 pixels.
The resulting image is a 100% crop. Because 1) it is at 100% zoom, and 2) it is a cropped portion of a larger shot.
When you post that, the primary purpose is to show sharpness (or lack of sharpness), or to draw attention to a certain area of a shot. It is not usually meant to be a final image for printing.
In THIS POST, the lower shot is a 100% crop.
Rad