From the Super Mario Wiki, the Mario encyclopedia
Triangular Blocks are magenta, triangle-shaped blocks with faces which are located at the bases of certain walls. They are primarily found throughout Dinosaur Land and first appear in Super Mario World.
If the Mario brothers run onto a Triangular Block, they will be able to continue their sprint by running up the wall that the block is placed against. This technique is required for getting over tall walls that they are unable to fly over. Yoshi is unable to use Triangular Blocks, but if he runs onto one, he will bounce high up. If a Koopa Shell hits a Triangular Block, it will also bounce up.
A prototype sprite sheet revealed in a 2017 interview revealed that circular and square variants of a Triangular Block were originally going to appear.
Triangular Blocks are also found in World-e of Super Mario Advance 4: Super Mario Bros. 3, more specifically in the levels Slidin' the Slopes, Sea to Sky, Castle Dash and Caped Escape. In addition to appearing at the bases of certain walls, Triangular blocks also appear between walls and ceilings, allowing Mario and Luigi to run up walls and onto ceilings.
Triangular Blocks appear in three episodes of the Super Mario World cartoon. One is first featured in "Ghosts 'R' Us", where Yoshi (who can, staying consistent with the games, only bounce on the block) briefly uses it to travel into Wizenheimer's haunted house and save Mario, Luigi, Princess Toadstool, and Oogtar. During a later episode, "King Scoopa Koopa", in an attempt to escape from the Chickendactyl-turned Cave People, Mario and Princess Toadstool run up a Triangular Block leading to a Star Road. In the episode "Born to Ride", Mario, Luigi, Princess Toadstool and Yoshi used a Triangular Block to drive up the wall in their motorbikes to avoid Duke, Lulu and Rockman. Rockman attempted to drive up the wall using the block, but failed and crashed (the block was also shown to frown as soon as Rockman failed).
Names in other languages
1 - In the dubbed version of the episode Ghosts 'R' Us, Yoshi inexplicably uses two different names at their plural to refer to a singular Triangular Block as he prepares to enter Wizenheimer's haunted house, despite consistently calling it a "triangle blockety" in the English version.