First appearing in Smash 4, the POW Block returns in Ultimate. Its game of origin is the original Mario Bros. arcade game where you use the block to flip the enemies upside down so you can defeat them.
In Smash Bros., its effect is similar. It can be either thrown or hit, and all the others on the stage will be stunned.
Which will cause a little damage but more importantly, it will upset their flow of movement.
While it's not technically my favorite item, its helped me out more times than I can remember.