Welcome to iMore! Auto-Brightness is enabled on your iPhone in Settings > Display & Brightness. This will automatically adjust the screen brightness based on ambient light conditions. You can turn it off if you want and then adjust brightness manually using Control Center.
If auto-brightness is on, the display on your phone with brighten or darken depending on the amount of ambient light available. Some people (lime my son) hate that and keeps his turned off and manually adjusts as needed. I personally like it on.