system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's happening in my brain when when I read an entire page or so of a book, only to realize that I wasn't actually paying attention and have no idea what I just read?
gpt
"Mind-wandering", an extremely common phenomenon, but understudied subject in cognitive and educational psychology. MW can take the form of task-related (e.g., imagining scenarios utilizing current material) or task-unrelated (e.g., dwelling on relationship issues, dinner planning, etc.). MW can be mitigated by reaffirming your internal motivations (e.g., interest in the topic) or external motivations (e.g., upcoming exams, being paid to absorb material). However, MW has also been correlated to higher creativity measures, so it ain't all bad.
Source: cognitive neuroscience researcher.

EDIT: Man_Fred_Beardman 's comment below is insightful for those interested in the cognitive mechanisms at play during mind-wandering:
"To expand a little on why you don't remember what you read, you have to understand memory is divided into three categories: Short-Term Memory (which only lasts approximately 4-7 seconds), Long-Term Memory (memories beyond 7 seconds), and Working Memory (memories being processed from short-term to long-term memory).
When you're reading and your mind becomes preoccupied with something else (like day dreaming/mind wandering) your working memory kind if shuts down (or at least focuses on processing your daydream into long-term memory instead of what you just read). So you end up remembering your day dream, but not what you read. Your short-term memory is still working, so you end up remembering the last few words you read, but nothing else."

EDIT 2: Thank you kind stranger for the gold!