Moss is a tiny non-flowering plant found in almost all lawns to varying degrees. Moss is a symptom and not the primary cause of poor growing conditions. Damp, poorly drained/compact soil especially in shade, underfeeding and undercutting or cutting the lawn too low and over-acidity can all contribute to the encouragement of moss in lawns.
Cultural improvements - Aerate the lawn to improve drainage and compaction. For shadey lawns consider pruning back any over hanging branches or reducing any shrubs etc that are blocking out light from the lawn.
Currently Bayer Garden do not produce a moss killer for lawns.