What it means
Hari is Vishnu, the one who takes away sorrow; Om is the pranava. One is God with form, the other beyond form, and this little phrase holds both.
Where it comes from
In the ashrams of Rishikesh and Haridwar, sadhus and visitors greet each other with Hari Om, and teachers like Swami Sivananda did a great deal to spread it. It's short enough to keep running in the mind all day.
How to chant it
With the breath: Hari breathing in, Om breathing out. With a mala or without.
When: Any time.
Try it now
The japa counter opens with this mantra selected and keeps count for you.