You will have to provide more details on your hardware and the process used for a specific answer, but most likely you cloned a smaller drive to a larger drive and did not select the option to resize the drive to fill unallocated space. So the program simply cloned the old small drive to the new drive and left some of the drive as unallocated space.
You may be able to use disk management to expand the partition into the remaining unallocated space if you are running Windows.