Its a point and shoot tool. You just point the red dot at the center of the camera screen to bottom of the object. It uses height of the camera and camera angle to find the distance

Distance = Camera height/tan(Camera tilt angle)

Obviously accuracy of your reading depends on you accurately specifying the camera height. Also a greater camera height would increase the accuracy. So, please

(1) For higher accuracy, measure the distance while standing up, not sitting down.
(2) Point the red dot at the bottom of the object, not at the middle of it.
(3) Pick your actual height.
(4) Slide the phone icon on the screen that represents the actual phone position with respect to your body.

One can use this tool to measure the height of the object too. Measure the distance pointing at the bottom of the object first and then click at the top button. Then point the dot at the top of the object. Again simple trigonometric equations. Nothing fancy. But a great tool if you follow the instructions.