Dirt Stairs are building-blocks cut into an L-shape that look like they are made of Earth.
Usually placed as stairs they will smooth the player-character's movement up and down gradients so no jumping is needed. They can be placed as projections of walls or for other purposes too when being rotated by pressing and holding R (as the default key) while moving the mouse.
Dirt Stairs can be crafted in your Crafting Menu (default key "Q"), but only after the crafting recipe has been unlocked by creating Dirt Slabs from blocks of common Dirt by simply putting them into a Processor. No crafting recipe has to be unlocked for that. Rocky Dirt cannot be turned into Slabs.
To craft 4 (blocks of) Dirt Stairs at once, you'll need