It is to accept, what is happening, to accept the other with his/her differences, without wanting him/her, as you are or as you would like him/her to be, to love without binding identifying, enslaving.
In the practice of yoga, letting go is accepting to negotiate with his/her lack of flexibility, to accept that the perfect posture is one that cannot be improved with his/her means of the moment.
Letting go is humility, simplicity the state without desire.
« Without desire, one is without thought; without thought, we are in reality». Patanjali