Idiom: to zone out
Tim: How was your history class today?
Chris: Well, I didn't sleep well last night, so I was so tired. I pretty much zoned out the whole class.
Tim: You still look a little sleepy. You'd better get some good sleep tonight.
Sara: Hey, Jill! Focus on the road!
Jill: I'm sorry. I was zoning out.
Sara: What were you thinking about?
Jill: My boyfriend and I have been having problems and I guess I just started daydreaming.
Meaning: to zone out means that someone cannot concentrate and their mind starts to wander or daydream.