If this question pops up in your head a lot and you are in a relationship then you may have some work to do on your communication skills with your woman.
Women and men are different when it comes to feeling validated in a conversation. Men don't always need to be validated directly but women, for the most part, do. It's a fact and if you accept it and understand it then you will avoid many fights and misunderstandings that will take place otherwise.
If a woman says something to you then she is looking for acknowledgement from you that you heard her and understand what she has said. A woman doesn't just want it, but she needs some sort of acknowledgement and validation from you that you heard her so that she can be satisfied that you heard what she said.
Everything that a woman says to you is for a reason. She either wants you to understand what she is feeling, what she needs done, what she would like you to do, or what she has experienced in the past.
Women know this very well when talking to other women and that's why a woman who wants to piss another woman off will not answer her when she asks a question or acknowledge her when she makes a statement. Watch this behaviour between women who don't like each other and you will see that it's true.
So if you are doing it to her then you can see how she's going to be pissed off that you haven't heard her or chose not to acknowledge her.
She will most likely say it again in a louder and upset tone or she will ask you if you heard her. The way you answer this question will allow her to receive validation that you heard her or make her more pissed off. So handle it correctly!
Let her know that you heard her and you were just processing the information. Then respond by answering her question or by repeating what she said. She will feel better and the problem will be averted.
The better solution is to respond when she talks to you the first time either with a verbal response or by making some sort of eye contact with her. Eye contact can take the place of verbal communication as long as it is followed with a nod of the head. If you just stare at her then she will still wonder if you heard her and start to repeat her previous statement again.
And remember, if you nod your head while she's looking away then she is obviously not going to see it and you will soon be hearing it again in a louder tone. So don't waste her energy for having to say it again and don't waste your energy that will go towards having to let her know that you did hear what she said and validate it to her after the fact