I think usually they are referring to “undivided attention” because technically, even though we’re pretty bad at it, you can do multiple things at the same time. I can drive, listen to music, and have a conversation but it starts to overload my brain at that point, as most of my attention is focusing on driving.
If we think of our brains & attention-span as RAM (memory), then some tasks take up more memory, and we have a limit to the RAM, so there are only so many processes that can run at the same time.
Advertisers/products/lovers, they’re all fighting for a bigger slice of your RAM.