Full frame sensor camera are taking the full image , example if you having A 50mm lens and having a 6d full frame camera body, then the picture will be exactly taken on 50mm. Taking same lens spans attaching to a crop body, like 7d , 7d m2, you will get a crop picture.
basically, every lens which you attach to a crop body, will have a small zoom, multiply 50mm by 1.6x and you will get 80m. Very useful for wildlife and birding.