How to 'Let Go' in Meditation?

> "And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to **let go**, attains concentration, attains singleness of mind. > > [Indriya-vibhanga Sutta (SN 48.10)](https://www.accesstoinsight.org/tipitaka/sn/sn48/sn48.010.than.html) What is exactly meant by letting go & how is this exactly done (in daily life & meditation) ? Why is the breath not the 'object', if it's called Mindfulness of Breathing?