It's almost certainly a hugging pillow or bolster or side pillow or whatever translation you want to use. I believe the Vietnamese term is gối ôm but it's a common piece of furniture in East Asia. I think you get a basic sense of its use there: it's used to prop up arms or lean against, especially while sitting/lying on the floor or another flat surface like a platform. You can still buy them and they're still very common in East Asia. You can get them in the US too. In cultural terms you can think of it as equivalent to someone lounging on a couch.

Beyond that... uh... what do you want to know? I could fill a book talking just about home furniture. Probably multiple books. Like... do you want to know about the expectations of Vietnamese elite furniture? Do you want to know how furniture was made and moved in East Asia? How the design itself changed in various times and places? How it became a fetish object in some subcultures?