In canon cameras, what is the thing that previews your image before you take the photo?

Before you take the photo, on the LD screen you can see the brightness of the photo change as you change the shutter speed and aperture. I have noticed my sisters canon camera has this but her nikon camera doesn't. Is it something to do with the lens? Can i turn it on in the settings in nikon? And what's it called? The cameras are canon 550d and nikon d7000

I don't know

Probably liveview, but that's only an approximation to the actual image and is easily affected by ambient light so I wouldn't rely on it for any accuracy.

You mean on the LCD in LiveView mode? All dSLR's with LiveView has it. It's not limited to Canon. Check the manual or menu system. Look for Bright/Dark area indicators in LiveView mode.

If the brightness changes only with the aperture and not the shutter speed, it's called "Depth of Field" preview.

On the D7000 it is located on the front of the camera next to the lens, on the upper right side - when viewing the camera from the behind. You normally depress it with your right index finger.

Look in the manual for specific details as to where it is located. Also, on Nikon cameras, you can re-program this button to do other tasks, so if it doesn't work, someone could have programmed it for a different function.

If it changes brightness with both shutter speed and aperture, it is most likely LiveView. In that case, look for the button marked "LV" on the back of the camera.