Grass Stairs are building-blocks cut into an L-shape that look like they are made of dirt with a bit of green Grass on top.
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.
Grass Stairs can be crafted in your Crafting Menu (default key "Q"), but only after the crafting recipe has been unlocked by creating Grass Slabs from blocks of green Grass by simply putting them into a Processor.
To craft 4 (blocks of) Grass Stairs at once, you'll need
Grass Stairs will not turn blocks of Dirt into blocks of Grass when placed next to them.