1.6 x focal length equals the focal length you would need on a FF body to get the same Image scale and field of view (magnification)as a 1.6 crop body.
1.6 x 100mm = 160mm, a 100mm lens on a crop body is equal to a 160mm lens on a FF Body.


Or if you have a FF body, focal length divided by 1.6 is equal to the focal length on a 1.6. Crop Body.
200mm / 1.6 =125mm, a 200mm lens on a FF is equal to a 125mm on a 1.6 crop body


IOW, with a FF body you would need a longer lens to get the same image scale and field of view that you could get with a crop body using a shorter lens.