Hi oliver8899!
Great question - there are a few different reasons this could be happening. Mainly this screen appears when your device gets too warm and it needs to cool down to normal operating temp's to reopen the device.
This usually happens if the device is left in direct sunlight on a hot day or in an environment where the phone can't dissipate heat fast enough to cool down.
If your seeing this message on your device screen while charging the device or when the above doesn't apply there may be other issues related to hardware on the device.
- here's a related apple article with more information: https://support.apple.com/en-us/HT201678
- to get a better answer if this is the case I would recommend scheduling an appointment at an apple store or authorized service provider through getsupport.apple.com or contacting apple support for more assistance.
hopefully this help!