1:1 is 1 pixel of your image equalling 1 pixel of your monitor.
generally on my screen, i'm only seeing the picture at 20% of it's original size and uncropped, but a lot of pixels don't show up as there aren't enough pixels on the screen to show it, the result is a compressed image in a way
at 100% there are no extra pixels to uncompress, so 100% is as much detail as it will ever get. if you zoom to 200% your screen is just using more of it's pixels to show you 1 pixel from the image and no benefits can be gained from this.
when you do a 100% crop, and cut the excess out of the image, what you see is what you get. you don't have to see the entire image, just what you need to show.
when someone says "here, this is a 100% crop for your viewing pleasure" all that's happened is someone has zoomed in on a picture to it's maximum resolution of 100% without going over, and provided you a small sample of the original picture at it's full size.
is that helping?