That's face recognition - the phone software is telling you that it recognizes that it's seeing a human face (or what looks like one). Check the camera app - there may be something in its settings to turn that off. It depends on the app.
I've already been through the camera app setting several times, there's nothing there about face recognition. I've checked and double checked and there's nothing there. this is concerning a samsung galaxy S8. I've also been trawling google but can't find any way to turn this yellow circle off.