In your lawn is overly shaded and/or has poor drainage, moss may be a problem.  Moss is a fast growing, shallow rooted plant that covers the ground, smothers grass and exhausts food reserves from the soil.

When moss appears in a lawn, it is usually because growing conditions for turfgrasses and landscape plants are adverse, usually too little light and air circulation.  Other conditions favorable for moss invasion include soil with poor aeration and percolation, compaction, low fertility and high acidity or alkalinity.