It is the feeling of being not seen, not heard.

Here is a silly example of what I experience:

I am reading the book in the room with my husband. I read him an interesting point the book made about the role of music. He asked what it meant. I discussed it a minute more. Meanwhile, he was cooking at the stove. I went on to discuss for another minute about the part right after in the book and how it pertained to our problem (him and me together dysfunction). He said nothing, was cooking. I realized I was going on, talking by myself. It was obvious by his lack of response, that he had shut down and was not interested in the conversation, which is about my recovery albeit recovery from a toxic relationship with him. So I said I see he’s not interested and stopped talking. He thought I was angry. I told him I wasn’t. But, he then said that he was silent because he was thinking about the role of music and what that meant. That’s why he didn’t speak. He was stuck on that. I asked if when I was speaking the second time, did he hear me, was he aware I was speaking while he was in deep thought? He said he knew I was speaking but he was thinking of something else- and that I should have known he was thinking. I should have given him more time to respond. I should have asked him if he was thinking. I should not have thought he was disinterested but should have known me was thinking. (Now this gets me upset normally, but not anymore since I understand) I say I see a man cooking who is silent. That behavior signals to me that you are not interested in having a conversation and I am not wrong to get that message. This is gaslighting that he is telling me my perception is wrong when it isn’t. It is his poor communication that is wrong. He is the mind reader who thinks I know he is thinking, good grief!
See this crazy-making paragraph I just wrote? I know how awful it is to even read this!
This little story is an example of how and why I do not feel seen and heard, it is destabilizing having it chronically done to me and gaslit that my perception and emotions are wrong by someone who is communicating in effectively (or just lying and gaslighting because he did not seem interested in talking about our issues and I know that he is not a partner in my recovery, only I am my own champion with a little help from my therapist and you kind souls.
Anyway, I effectively communicated this time without any fighting asking him to please speak up if he is reflecting and to not let me monologue while he tunes me out.