Camera app in iPhone comes with a tap to focus feature. You tap the screen on subject you want to focus before the capture.
Unless you have an object that is very near the camera like the flowers in the above picture, most of the time all scene in the picture will be in focus. This is due mainly to the inherent limitation of optics for camera in smartphone. So for most of the scene especially in landscape, there is not much changes in focusing you can notice by using tap to focus.
But tap to focus will also trigger the iPhone camera app to adjust the exposure of the picture. If you have a high contrast scene, tap to focus on the darker area will brighten the exposure. And tap to focus on the brighter area will darken the exposure. Try out this tap to focus feature when you capture your next high contrast scene.