How can I reduce the hard disk space occupied by "Other"?
Here is an article explaining what the "Other" category of files is, from Apple support. Basically, the Other folder is anything that cannot be recognized as Audio, Video, Photos, Apps, and Backups. If you want to free up some space in here, you need to navigate to your home folder, and move any files to the Trash that you don't need. Then, empty the trash. If there are still files taking up a lot of space that you really need, you can compress them. (Create a folder, place the files in the folder, and click "Compress ." You can also transfer some files to an external drive.  

Also, here is an iGeeksBlog article giving some more tips on how to remove things from the Other folder.