How to 'Let Go' in Meditation? &quot;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)

What is exactly meant by letting go &amp; how is this exactly done (in daily life &amp; meditation) ?
Why is the breath not the 'object', if it's called Mindfulness of Breathing?
Great question, you hit the nail on the head with this question.

The goal of Buddhist path is liberation of mind. Liberation of mind is cessation of grasping and attachment. Cessation of grasping and attachment is letting go.

So when you meditate, you look at your mind, you see grasping and attachment (it is easy to detect because it always creates dukkha, the painful feeling of wrongness) - and you let go of it to gladden and liberate your mind.

Mindfulness of breathing (and of body in general) is a tool for seeing your mind, because the state of your mind is always reflected in the breathing and the body.

Starting from the most coarse and then progressively smaller stuff.

First, you let go of erratic behavior (violence, theft, adultery, intoxication, non-factual or divisive speech), then you let go of major cravings and aversions (incl. social), various conceptual attachments &amp; stereotypes, then you let go of negative self-judgement, then let go of doubt - etc. keep going smaller and smaller until nirvana.