iPhone icon
What is the small icon in topic top right corner of home page next to percentage of charge that looks like a lock inside of a circle with an arrow at the end?
iPhone 6, iOS 12
That means you have screen rotation locked. When you have screen rotation set to ON (Locked) then you screen will not rotate when you rotate your iPhone with supporting apps. Not all apps rotate with the iPhone.
If you would like to remove the rotation lock then please read below:
Lock or unlock the screen orientation
You can lock the screen orientation so that it doesn’t change when you rotate iPhone.
➔ Open Control Center, then tap 
.
When the screen orientation is locked, 
appears in the status bar (on supported models).
Axel F.