If you have a lot of photos and/or videos, they are taking a lot of your space. Go into Settings - General - Manage Storage. Then you can see what is using up most of your availability.
If you have an iCloud account you can move the photos into there.
Be careful though. I just lost a whole load of vids by thinking I'd moved them and deleting them when I hadn't. Or, you can download Dropbox for free and move your photos into there. Very easy.