Disturbing picture. It likely suggests this young lady has had to put up with lowlifes trying to take advantage of her in the past.
When safely commuting requires such a device, it is time to re-think where you live and work. Shouldn’t be that way, but the scumbags she’s worried about are not likely to ever learn class, manners, or respect.
That’s just a piece of an art project the girl was transporting on the train. People saw it and thought it was funny, so they posted on the internet it was her “personal space cube”, but that was just made up.