The iPhone's touch screen gets easily mucked up with the natural oils in your fingers, which can build up over time. This grease needs to be wiped away periodically in order to keep your device clean.
Unplug all cables from the iPhone and turn it off.
Wipe away any grease from the touch screen and surrounding area with the cloth that came with it.
Dampen a lint-free cloth with water if the cleaning cloth that came with the iPhone is not available, and wipe away any grease from the touch screen and surrounding area.