"Sodhmantra is external medicine and Bhavana is internal medicine." What is the concise meaning?

I would like to ask you about what the old ancient Thai people used to say regarding Sodhmantra (general Pali term for the chanting of the Lord Buddha’s Dhamma) and Bhavana (Pali term for actual meditation): “Sodhmantra is external medicine and Bhavana is internal medicine.” What is the concise meaning?

The Middle Way Meditation Retreat // May 18-27, 2016 - Vietnam

From May 18th – 27th, 2016, The Middle Way Meditation Retreat under the leadership of Venerable monk Piset Jittasuto, Vice Deputy Chief of the Middle Way Meditation hosted the meditation retreat.