Manual Focus allows you to over ride your camera's auto focus to achieve the desired detail in every shot.
Using the slider in the manual focus function, you will be able to see your image coming in and out of focus. This is a handy feature when shooting an image you would like to get very close to an object, or when trying to mimic macro shots.
Tap on 
to engage the manual focus control.
Using the slider, you can adjust your focus.
Tapping 
again will bring you back to automatic Focus.