Can someone explain how being lost in thought can disable what you hear or see or sometimes enhance it?

Yesterday I was sitting in an office and I had several things on my mind. The receptionist said something but at first I wasn’t sure it was directed to me. Then when I did, I was still tuning her out. I know she was saying something but I only caught the tail end of it and this was after I had asked her to say it again. It was as if I needed to hit a switch to actually listen and understand.
Thinking back the same can happen in reverse. Someone can be speaking and I am able to understand what they are saying and I’m fully aware of everything while still processing other thoughts.
So I’m hoping someone can help me understand why this happens or even if that is how everyone brain works or just mine. Thanks.