How did you move the files? If you copied the files to an external drive first and then deleted them from your Mac, you must empty the Trash with the external drive connected to reclaim the space.
If that's not the case, or if emptying the Trash doesn't free up space, use iBoysoft DiskGeeker to check the files on your Mac. With a scan, it can discover all files on your Mac, including hidden ones, and display them in descending order by size. This way, you can check whether the files are actually deleted and efficiently remove other unnecessary files that are cluttering your Mac.