What is this yellow square called? It is from an iphone camera?
What is this yellow square called? It is from an iphone camera? 
Isn't it a symbol for backlighting?
It's just the area that the camera is focussing and metering on.
Most cameras have independent metering (to set exposure) and auto focus zones.
Generally auto mode, cameras use matrix metering (they measure the light of the entire frame and adjust the exposure for it) and multipoint autofocus which detects the closest object to the photographer.
If you want a photo a specific way, on a normal camera you set it to centre weight or spot metering and select a single autofocus point. This involves the complex process of pointing the camera directly at what you want to have perfectly exposed, pushing a hold button, and recomposing the shot so that object is in the correct part of the frame under the autofocus point you want to use.
The iPhone and other cameras make it easy since they assume where you touch on the image is important to you, so it adjusts both focus and metering to make it look good.
When you tap on the screen when you are on the iPhone camera app, it's purpose is to focus on objects near to you or further away from you. If the yellow square is still on the screen you are able to raise the live picture/video brightness by scrolling your finger up or down.
That's to adjust the ISO (brightness) of the shot. The box itself also is where the tap to focus is set on.
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