Probably one or more of the apps on your iPhone is using this 'other' storage to store its files/data.
Try this:
- Open the Settings app.
- Tap General, Usage, and wait for the Storage list to display.
- Identify any apps that are using a large amount of space.
You can then open the app(s) and trash any unwanted data, drawings, music, video, whatever...
Good luck!
RTP.