I first heard this term from a Hakomi therapist.
This is how it works: Perhaps when you were young, people around you weren't present. Or perhaps there was a time when you expressed how you felt to someone or shared what you thought, and those words were not received in a positive way. Perhaps your experience was denied or brushed off. Or perhaps you were given some kind of grief – for essentially being who you were.
A part of you might have felt bad or shut down. But I think that we never lose the potential to come into our full selves; we just need a supportive situation in which we are allowed to grow.
In therapy, you get the opportunity to say what's true for you, even if that means expressing anger or frustration with the therapist. And you won't be shamed or given hell for it. In fact, you'll be heard and validated, and that feels good. Something gets healed. That's the missing experience.