I don't think you have to have firsthand knowledge of something to describe it, but I also don't believe you can necessarily describe it even if you have firsthand knowledge of it. there's a bunch of stuff I have firsthand knowledge of but cannot describe. There's also a bunch of stuff I could describe based on what I hear others say it is. I agree with both mastadon and stopdog here in different ways...
as for what trust is? For me, it's a hope that the person will be there for me and react in expected ways based on history. Do I actually trust anyone fully? Nope. I trust aspects of them, and I trust in certain promises, but there's no one I truly trust in the full sense of the concept. Part of it comes from humans being humans... my animals I trust completely, but part of that comes from a trust in myself to be able to read them and know how they react according to history and current signals...