A lower f-stop number means that the aperture is bigger (relative to the focal length), such that it let's in more light. This has several subjective benefits: more light can be used for shorter exposure time or lower ISO (sensor gain) each leading to a sharper image. Also, it creates a shallower depth of field or "more blur" which separates the subject for a more pleasing picture. (This may be irrelevant on sensors as small as on a phone, which counteract shallow depth of field effects.) It may reduce the image quality if the lens of of lower quality, as it is more expensive to build large-aperture lenses.