An crop is when you take a picture, lets say of your house, and you select a small area of your picture, lets say a door handle, and you remove the rest of the image.
So, lets say your original image of the house is 5000 x 2500 pixels, which does not fit on your computer screen, so when you view it, you might view it at 25% or something like that, unless you set the zoom at 100% and only view some of the image.
Now, lets say your door knob is in that picture is 200 x 200 pixels, you can cut that out from the picture, and make a new file with it. That new 200 x 200 pixel file fits perfectly in your computer screen or a website, and is called a 100% crop. This way, you can show the quality of the image while viewed at 100% zoom, without having to share a 8mb picture that's 3 or 4 times bigger than your computer screen.
The reviewers show 100% crops to give you an idea what it would look like if you view the image at 100%. Lenses have different image quality (IQ) at different places of a picture, so Camera Labs website shows you the full pictures scaled down, so you know what you're looking at, and then shows you the 100% crop in the center of the picture, which is where the lenses perform best, and then near the border of the picture, where the lenses perform worst.
I hope that helps!
Cameras: Canon EOS Rebel T2i, Canon S90
Lenses: Tamron: SP70-300mm F/4-5.6 Di VC USD, Canon: EF-S 18-55mm f/3.5-5.6 IS Lens, 75-300mm f/4.0-5.6 III, and EF 50mm f/1.8 II
Retired camera: Fujifilm Finepix s700