Zoom is a feature common among cameras and is used to make the subject appear closer. Cameras on mobile phones often have a zoom feature as well but most often it is digital zoom.
Digital zoom is implemented in one of two ways:
Cropping - the software crops the image so that the subject would appear bigger on the screen of the phone but the resulting image is smaller than the maximum resolution of the camera. The photo of the subject does not have any more detail than a non-cropped photo would.
Stretching - this is similar to cropping but instead it stretches the cropped photo to the selected resolution. Since the stretching is done by an algorithm that uses just the information from the cropped photo no additional detail is visible.