Pillows look like double-thick carpets and are crafted with wool using the "slab" recipe shape (three wool in a horizontal line in crafting table.) They can be placed on any surface that a carpet can be placed on.
Once placed, pillows become seats. The Use action makes the player sit on the pillow, same animation as sitting in a boat. Sneaking (in Java) or Jumping (in Bedrock) allows the player to stand up again.
This allows players to build better chairs. Actually including a chair object would limit creativity, but using carpets or stairs currently doesn't allow actually sitting down, even though the sitting animation is already included in the game. Adding a pillow as, basically, a non-mobile boat solves the problem,
Pillow on fencepost = barstool
Pillows on line of fenceposts = bench
Pillow on wooden plank = footstool. Add signs or trapdoors in various configurations to make armchair
Pillows on fenceposts arranged in square, stacked trapdoors on one side = straight-back chair
Pushing a villager or other mob makes the mob sit down, exactly as for pushing the mob into a boat. This allows players to create interesting, lively scenes.