It's when you truly believe, even know, that the other person won't hurt you, either physically or emotionally. It's when you know the other person has your best interest at heart. It's when you know that the other person will be there for you and support you. When the other person will do what they say they will.

I think nearly everyone has some level of trust in them. You'd never leave the house or drive a car or walk across a road otherwise. You'd assume that everyone out there is actively trying to kill you.