Cropping and masking are both terms that describe hiding part of an object. In general, the difference is that cropping uses a rectangle to trim the edges of an image, and masking uses an arbitrary shape to make an object’s background transparent. A common example of a mask is a clipping path, which is a mask made for a specific image.
Use graphics frames to crop or mask objects. Because an imported graphic is automatically contained within a frame, you can crop or mask it immediately without having to create a frame for it. If you haven’t created a frame for an imported graphic manually, the frame is automatically created at the same size as the graphic, so it may not be obvious that the frame is there.