It was originally developed for borderline personality disorder but it used all the time now for everything.

An Overview of Dialectical Behavior Therapy | Psych Central

Radical Acceptance Part 1

The fact is your Dad got married and he had every right to. There is absolutely nothing you can do about it or could have done about it. It is his life. You have no control over the thoughts, feelings, and actions of others. Facing the reality is radical acceptance. Once the fact is faced and accepted then you have a choice in how you choose to deal with it. I think your therapist is saying that your anger and feeling of betrayal is causing you to suffer. You can choose not to suffer because of it by accepting his decision and forgiving him for it.