From the Super Mario Wiki
A Yellow Block.
Yellow Blocks are objects seen in the Paper Mario series; they can be destroyed with the basic hammer. In Paper Mario and Paper Mario: The Thousand-Year Door, they are not very hard to get by, with Stone Blocks and Metal Blocks presenting more difficult obstacles. In the latter game, however, a new type of Yellow Block is also present, being much larger, having angry eyebrows, and requiring a swing from the Super Hammer in order to be destroyed.
In Super Paper Mario, Yellow Blocks are significantly rarer than in the previous games, likely due to the fact that Cudge, the hammer Pixl, is not acquired until Chapter 5.
Yellow Blocks look similar to Rotating Blocks from Super Mario World.