When you delete videos or photos from your iPhone they aren't removed instantly from your device

Scroll to the bottom of the Album tab in the Photos app and you'll see an album called Recently Deleted in Other Albums

Photos and videos can remain here for up to 40 days before being automatically removed

You can see how many days a photo has left before it will be deleted permanently

By selecting items you can choose to Recover them, or Delete instantly if you're running low on storage space