Weird loading screen popped up

Weird black screen popped up, with a loading percentage on top and a decline and accept button on the bottom.

It appeared first when the screen was locked with it loading at 21%, and then after unlocking screen at 33%. As I didn't know what to do I turned off wifi and didn't have time to take a snapshot, and then it just stopped appearing. I turned wifi on again but it still didn't pop up again. Anybody know what it is?! If tried to find something about on the internet, but nothing! Would really appreciate some help!

THanks!!