Other is everything that is not Apps, Photos, Movies, Audio, or Backups.
Generally when people have a very large Other on their startup drive, the Spotlight index is corrupt. The Spotlight index is used to create that display, so if it is not working correctly, you will see odd information.
Try re-indexing Spotlight before going down the path you are on: Spotlight: How to re-index folders or volumes - Apple Support
Once done, a program like Grand Perspective, Disk Inventory X, or Daisy Disk which all will graphically display where all your storage space is allocated.
The primary things you'll want to remove are your Movies and Pictures. Those are generally the largest users of space. If you wish to keep them, move them to another hard drive or burn them to disk. Make sure you back up that other disk, too.