What it means
Har is a name of Shiva, the one who takes away sorrow and wrongdoing. Mahadev is the god of gods. Together they make a call, meant to be raised with feeling.
Where it comes from
In Banaras it doubles as a greeting; people meet and say Har Har Mahadev. In Sawan the kanwariyas carrying Ganga water shout it mile after mile. It was also the war cry of the Maratha armies. This isn't a mantra you count quietly on a mala. Its nature is to be called out.
How to chant it
Out loud and wholeheartedly, ideally with other people. It's customary on entering a Shiva temple and at the close of an abhishek.
When: Sawan, Maha Shivratri, and any time you walk into a Shiva temple.
Try it now
The japa counter opens with this mantra selected and keeps count for you.
Questions people ask
You can, but its real form is the call. For mala japa, Om Namah Shivaya suits better.