Why does a transaction have the fields
attachmentTimestampUpperBound. What does it mean for a timestamp to have bounds?
You have no way of knowing when exactly a transaction was attached to the tangle (because you can even set this value), instead you can have a max interval you can trust, maybe giving a example you be easier to understand.
I am at home with my family, I decide to go the store to buy some icecream, so I leave my house at 9:00 AM and they see it. I buy the Icecream at 10:00 AM and arrive at home at 11:00 AM. I say to them I bought the icecream at 9:30 AM, is that true? No, but they can know the possible time interval I bought it. The 9:30 AM is the timestamp they see, 9:00 AM is the attachmentTimestampLowerBound and 10:00 AM is the attachmentTimestampUpperBound.