To let go means to give up coercing, resisting, or struggling, in exchange for something more powerful and wholesome which comes out of allowing things to be as they are without getting caught up in your attraction to or rejection of them, in the intrinsic stickiness of wanting, of liking and disliking. It's akin to letting your palm open to unhand something you have been holding on to.
Jon Kabat-Zinn, Wherever You Go, There You Are: Mindfulness Meditation in Everyday Life
acceptance, let-go