Hugin Crop tab
Use the Crop tab to indicate the area of the panorama that should be used as output. This crop tab doesn't work on the individual images, but on the entire panorama.
The "Autocrop" and "HDR Autocrop" buttons will adjust the crop rectangle so that it is entirely within the image area, i.e. there will be no 'black' borders in the final stitched image. It does this by maximising the area of the rectangle rather than the width or height. This is automatically done when running the Hugin Assistant tab.
To change the cropping at each edge, move the mouse towards that edge until a semi-transparant white box appears along it, then drag with the left mouse button until the edge is where you want it. The area outside the dragging rectangle represents the areas that will be cropped off the panorama. You can move two edges at once by moving the mouse towards the corner shared by the edges until both white boxes appear. If you wish to move the whole region at once, move the mouse into the middle so that all four edges have boxes along them and drag.
This Hugin Crop tab also has the option to display guides in the preview to help you crop the panorama until the composition applies to for example the "rule of thirds".
Note: cropping the panorama shouldn't be confused with a camera's Crop factor, which is something else entirely.
Note 2: This Hugin Crop tab is neither to crop individual images, like you would do for e.g. fisheye Projection images having a circular area in the middle with a useless black area outside, or scanned images that might have edges that need to be cropped away.