To let your frustratoins, anger, etc. go and not let it stress you out like it has been doing. Letting go means you don't have to stress over whatever you're letting go anymore.
The feelings tied with what you're letting go may still hold a place in your heart, but it is better to let whatever it is go and get on with your life than to let it hold you down and depressed.
They let each other go because they knew it wouldn't do any good staying together. It was a mutual break up.
They've moved on
and are living happier lives now. I'm sure they're both happy they let go that stressful relationship.
*...and they lived happily ever after. fin*