"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?