If you want to live a happy, healthy and fulfilling life then this is true. Resentment manifests in the physical body in all different ways that will eventually affect your health. We need to forgive all the time, everyday and for our entire lives.
Forgiveness is being able to fully empathise with another’s actions and then completely letting go. It doesn’t mean you condone the behaviour, rather you choose to move forward with no energetic ties attached.
You don’t need to know how to forgive, but if you want to forgive then the universe will show you how. But you need to let go of all associated feelings and thoughts to be aligned with what you need to see.