From the Super Mario Wiki, the Mario encyclopedia
Directional Blocks are objects that appear in Super Mario 3D Land. They are purple blocks with an arrow on each side. When Mario/Luigi hits one from below, it will move in the direction it points to for a distance that is roughly half of its height/width, which usually allows him to access certain blocks or collectibles he cannot reach with a normal jump. Their positions can only be reset by replaying the levels they appear in.