system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening in the brain when I just 'zone out' of a conversation with someone and get a glazed look over my face?
gpt
I am not a specialist on this topic, but there may be a reason behind in a form of brain focus. Your brain never perceives everything in once, otherwise it would get overwhelmed and would not be able to process either signal properly (i.e. not being able to make sense out of anything seen, heard, felt, etc.). This means it has to filter some elements out, those which the brain deems "not important".

This is particularly useful when, for example, walking on a street. You don't pay such an attention to that small fresh bubble-gum on the sidewalk, you notice small text much less often, etc.

As it seems your brain just sometimes feels the conversation is not that important and indulges your mind into thinking about something else. Imagine talking to someone about his car, suddenly your brain shifts your mind into thinking about your car insurance, which is going to expire soon ... and bam! You have zoned out. 

Either that would explain it, or you may have a problem with concentration (maybe a mild form of ADHD, that is).