The presence of moss can be due to a few factors. Low competition from vascular plants is usually the main reason which can be the result of a number of underlying factors. Mosses are good stress toleraters but often poor competitors. In a lawn you can have areas of moss rather than grass due to heavy shade, low nitrogen, high acidity or alkalinity (which reduces availability of nitrogen), very moist or very dry soil or any combination of these. You may be able to grow ferns, hostas, hydrangeas or rhododendrons in these areas even though grass does not.