Why Hindus say 'Hari Om'? What does it signify?

It is an utterance by Hindus seeking protection from the Lord. Om is a basic monosyllable mantra used to invoke Cosmic energy, where as 'Hari' is the Supreme Power that removes all the causes of sorrows and sufferings.It is also 'ajapa japa', which means 'a prayer without being prayed.' It comes normally, effortlessly and spontaneously, after a period of practice.